Magnetic recording medium, non-magnetic acicular black iron-based composite particles and process for producing the particles

ABSTRACT

A magnetic recording medium of the present invention comprises:  
     a non-magnetic base film;  
     a non-magnetic undercoat layer formed on the non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles; and  
     a magnetic coating film comprising a binder resin and magnetic particles,  
     the non-magnetic acicular black iron-based composite particles having an average major axis diameter of usually 0.011 to 0.35 μm, comprising: acicular hematite particles or acicular iron oxide hydroxide particles (core particles) having an average major axis diameter of 0.01 to 0.3 μm; a a coating layer formed on the surface of said acicular hematite particle or acicular iron oxide hydroxide particle, comprising a specific organosilicon compound; and a single carbon black coat formed on the coating layer in an amount of 21 to 50 parts by weight based on 100 parts by weight of the core particles.

CROSS REFERENCE TO RELATED APPLICATION:

[0001] This is a continuation-in-part of application, Ser. No.09/632,096 filed Aug. 2, 2000, which is a continuation-in-part ofapplication, Ser. No. 09/523,646 filed Mar. 10, 2000, which is acontinuation-in-part of application, Ser. No. 09/208,771 filed Dec. 10,1998.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a magnetic recording medium,non-magnetic acicular black iron-based composite particles and a processfor producing the non-magnetic acicular black iron-based compositeparticles, and more particularly, to a magnetic recording medium havinga smooth surface, a lower light transmittance, a lower surfaceresistively value, a low friction coefficient and an excellent runningdurability; non-magnetic acicular black iron-based composite particleshaving an excellent dispersibility in vehicle due to less amount ofcarbon black desorbed or fallen-off from the surfaces thereof, a moreexcellent blackness, a lower volume resistivity value and awell-controlled myristic acid absorption; a process for producing thenon-magnetic acicular black iron-based composite particles; and anon-magnetic substrate for magnetic recording media having a smoothsurface and a lower surface resistivity value.

[0003] With a development of miniaturized and lightweight video or audiomagnetic recording and reproducing apparatuses for long-time recording,magnetic recording media such as a magnetic tape and magnetic disk havebeen increasingly and strongly required to have a higher performance,namely, a higher recording density, higher output characteristic, inparticular, an improved frequency characteristic and a lower noiselevel.

[0004] Video tapes have recently been required more and more to have ahigher picture quality, and the frequencies of carrier signals recordedin recent video tapes are higher than those recorded in conventionalvideo tapes. In other words, the signals in the short-wave region havecome to be used, and as a result, the magnetization depth from thesurface of a magnetic tape has come to be remarkably small.

[0005] With respect to short wavelength signals, a reduction in thethickness of a magnetic recording layer is also strongly demanded inorder to improve the high output characteristics, especially, the S/Nratio of a magnetic recording medium. This fact is described, forexample, on page 312 of Development of Magnetic Materials and Techniquefor High Dispersion of Magnetic Powder, published by Sogo Gijutsu CenterCo., Ltd. (1982), “. . . the conditions for high-density recording in acoated-layer type tape are that the noise level is low with respect tosignals having a short wavelength and that the high outputcharacteristics are maintained. To satisfy these conditions, it isnecessary that the tape has large coercive force Hc and residualmagnetization Br, . . . and the coating film has a smaller thickness, .. . ”.

[0006] Development of a thinner film for a magnetic recording layer hascaused some problems.

[0007] Firstly, it is necessary to make a magnetic recording layersmooth and to eliminate the non-uniformity of thickness. As well known,in order to obtain a smooth magnetic recording layer having a uniformthickness, the surface of the base film must also be smooth. This factis described on pages 180 and 181 of Materials for SyntheticTechnology—Causes of Friction and Abrasion of Magnetic Tape and HeadRunning System and Measures for Solving the Problem (hereinunderreferred to as “Materials for Synthetic Technology” (1987), published bythe Publishing Department of Technology Information Center, “. . . thesurface roughness of a hardened magnetic coating film depends on thesurface roughness of the base film (back surface roughness) so largelyas to be approximately proportional, . . . , since the magnetic coatingfilm is formed on the base film, the more smooth the surface of the basefilm is, the more uniform and larger head output is obtained and themore the S/N ratio is improved.”

[0008] Secondly, there has been caused a problem in the strength of abase film with a tendency of the reduction in the thickness of the basefilm in response to the demand for a thinner magnetic coating film. Thisfact is described, for example, on page 77 of the above-describedDevelopment of Magnetic Materials and Technique for High Dispersion ofMagnetic Powder, “. . . Higher recording density is a large problemassigned t the present magnetic tape. This is important in order toshorten the length of the tape so as to miniaturize the size of acassette and to enable long-time recording. For this purpose, it isnecessary to reduce the thickness of a base film . . . With the tendencyof reduction in the film thickness, the stiffness of the tape alsoreduces to such an extent as to make smooth travel in a recorderdifficult. Therefore, improvement of the stiffness of a video tape bothin the machine direction and in the transverse direction is now stronglydemanded . . . ”

[0009] The end portion of a magnetic recording medium such as a magnetictape, especially, a video tape is judged by detecting a portion of themagnetic recording medium at which the light transmittance is large by avideo deck. If the light transmittance of the whole part of a magneticrecording layer is made large by the production of a thinner magneticrecording medium or the ultrafine magnetic particles dispersed in themagnetic recording layer, it is difficult to detect the portion having alarge light transmittance by a video deck. For reducing the lighttransmittance of the whole part of a magnetic recording layer, carbonblack fine particles or the like is added to the magnetic recordinglayer. It is, therefore, essential to add carbon black or the like to amagnetic recording layer in the present video tapes.

[0010] However, addition of a large amount of non-magnetic particlessuch as carbon black fine particles impairs not only the enhancement ofthe magnetic recording density but also the development of a thinnerrecording layer. In order to reduce the magnetization depth from thesurface of the magnetic tape and to produce a thinner magnetic recordinglayer, it is strongly demanded to reduce, as much as possible, thequantity of non-magnetic particles such as carbon black fine particleswhich are added to a magnetic recording layer.

[0011] Consequently, it has been strongly demanded to provide a magneticrecording medium capable of exhibiting a low light transmittance evenwhen the amount of carbon black fine particles added to a magneticrecording layer thereof is reduced as low as possible. From thisviewpoint, it has been strongly required to essentially improveproperties of a substrate therefor.

[0012] Further, in order to reduce not only the above-mentioned lighttransmittance of the magnetic recording medium but also an electricresistance thereof, there has been hitherto proposed a method of addingcarbon black fine particles to the magnetic recording layer.

[0013] The conventional magnetic recording medium to which carbon blackfine particles are added, is described in detail below.

[0014] When a magnetic recording medium has a high surface resistivity,an electrostatic charge thereon tends to be increased, so that cut chipsof the magnetic recording medium and dirt or dusts are attached onto thesurface of the magnetic recording medium upon production or use thereof,and as a result, such a problem that the number of “drop-out” becomesincreased, is caused.

[0015] In order to decrease the surface resistivity of the magneticrecording medium to about 10⁸ Ω/cm², a conductive compound such ascarbon black fine particles has been ordinarily added to the magneticrecording layer in an amount of not less than about 5 parts by weightbased on 100 parts of magnetic particles contained in the magneticrecording layer.

[0016] However, when the amount of carbon black fine particles added orthe like which cannot contribute to improvement in magnetic propertiesof the magnetic recording layer, is increased, the magnetic recordingmedium has been deteriorated in electromagnetic performance as describedabove, resulting in inhibiting the magnetic recording layer from beingthinned.

[0017] It has been required to further enhance the performance ofmagnetic recording media. The magnetic recording media have beenstrongly required to exhibit not only high-density recording propertybut also improved physical properties such as running property.

[0018] A good running property of the magnetic recording media can beusually assured by adding to a magnetic recording layer ordinarilyformed as an upper layer of the magnetic recording medium, a fatty acidsuch as myristic acid and stearic acid (hereinafter referred to merelyas “myristic acid”) in an amount of usually about 0.5 to about 5% byweight based on the weight of magnetic particles used in the magneticrecording layer. The thus added myristic acid is controlled so as to begradually oozed from the magnetic recording layer, thereby rendering thesurface thereof more slippery.

[0019] When the amount of the myristic acid oozed onto the surface ofthe magnetic recording layer is too small, it is difficult to impart agood running property to the magnetic recording media. On the contrary,when a large amount of the myristic acid is added to the magneticrecording layer in order to increase the amount of the myristic acidoozed onto the surface thereof, the myristic acid is preferentiallyabsorbed onto the surfaces of magnetic particles dispersed in themagnetic recording layer, thereby inhibiting resins from being absorbedonto the surfaces of the magnetic particles. As a result, it becomesdifficult to disperse the magnetic particles in vehicle. Also, theincreased content of the myristic acid as a non-magnetic component tendsto cause deterioration in magnetic properties of the obtained magneticrecording media. Further, since the myristic acid acts as a plasticizer,there arise problems such as deteriorated strength of the magneticrecording media.

[0020] With the recent tendency toward reduction in thickness of themagnetic recording layer, an absolute amount of myristic acid beingaddable thereto is decreased. Also, in order to achieve high-densityrecording on magnetic recording media, it has been required thatmagnetic particles used therein are much finer, resulting in increase inBET specific surface area thereof. The increment in BET specific surfacearea of the magnetic particles increases an amount of the myristic acidabsorbed onto the surfaces thereof. As a result, it becomes more andmore difficult to assure a good running property of the magneticrecording layer, by controlling the amount of the myristic acid oozedonto the surface of the magnetic recording layer only by a amount ofmyristic acid added to the magnetic recording layer.

[0021] In Japanese Patent Application Laid-Open (KOKAI) No. 5-182178(1993), it is described that “in the present invention . . . , inorganicparticles and fatty acid contained in the non-magnetic layer areprevented from being interacting with each other to control respectivecontents of the fatty acid in the non-magnetic layer and the magneticrecording layer, thereby improving a running durability of the magneticrecording layer . . . ”. As understood from the above description, ithas been strongly required to assure a good running property of magneticrecording media by appropriately controlling the amount of myristic acidoozed onto the surface of the magnetic recording layer, by means of boththe magnetic recording layer and the non-magnetic undercoat layer havinga thickness two or more times that of the magnetic recording layer.

[0022] With the reduction in thickness of the magnetic recording layeror the base film therefor, various attempts have been conducted in orderto improve properties of a substrate on which the magnetic recordinglayer is formed, thereby enhancing a surface smoothness and a strengthof the magnetic recording medium. In this regard, there has beenproposed a method of forming on a base film, at least one undercoatlayer composed of a binder resin and non-magnetic particles dispersed inthe binder resin and containing iron as a main component, such asacicular hematite particles or acicular iron oxide hydroxide particles(hereinafter referred to merely as “non-magnetic undercoat layer”), andsuch a method has been already put into practice (Japanese PatentPublication (KOKOKU) No. 6-93297(1994), Japanese Patent ApplicationLaid-open (KOKAI) Nos. 62-159338(1987), 63-187418(1988), 4-167225(1992),4-325915(1992), 5-73882(1993), 5-182177(1993), 5-347017(1993) and6-60362(1994), and the like).

[0023] It is known to use as the non-magnetic particles for non-magneticundercoat layer, non-magnetic particles which surfaces are coated withhydroxides of aluminum, oxides of aluminum, hydroxides of silicon,oxides of silicon or a mixture thereof in order to improve thedispersibility thereof in vehicles, thereby enhancing a surfacesmoothness and a strength of the obtained non-magnetic substrate(Japanese Patent Nos. 2,571,350 and 2,582,051, and Japanese PatentApplication Laid-open (KOKAI) Nos. 6-60362(1994), 9-22524(1997) and9-27117(1997) or the like).

[0024] Also, it is known that in order to decrease an amount of carbonblack fine particles added to the magnetic recording layer and reduce alight transmittance of the magnetic recording medium, blackish brownacicular hematite particles or blackish brown acicular iron oxidehydroxide particles are used as the non-magnetic particles fornon-magnetic undercoat layer (Japanese Patent Application Laid-open(KOKAI) Nos. 7-66020(1995), 8-259237(1996) and 9-167333(1997)).

[0025] It is also known to use non-magnetic acicular particles such asacicular hematite particles and acicular iron oxide hydroxide particleson which surfaces carbon black is adhered in an amount of 1 to 20 partsby weight based on 100 parts by weight of the non-magnetic acicularparticles (European Patent No. 0924690 A).

[0026] Further, it is known that in order to reduce an electricresistance of the magnetic recording medium, a mixture of non-magneticiron oxide particles and carbon black fine particles is used as thenon-magnetic particles for non-magnetic undercoat layer (Japanese PatentApplication Laid-open (KOKAI) Nos. 1-213822(1989), 1-300419(1989),6-236542(1994) and 9-297911(1997) or the like).

[0027] At present, it has been most demanded to provide magneticrecording media having a smooth surface, a lower light transmittance, alower surface resistivity value and an excellent running property.However, non-magnetic particles for non-magnetic undercoat layer whichcan provide such magnetic recording media satisfying all of the aboverequirements, have not been obtained.

[0028] Namely, the magnetic recording media produced by using the aboveconventional acicular hematite particles or acicular iron oxidehydroxide particles whose surfaces are coated with a hydroxide ofaluminum, an oxide of aluminum, a hydroxide of silicon or an oxide orsilicon, as non-magnetic particles for non-magnetic undercoat layer,have a smooth surface, but fail to reduce a light transmittance thereofbecause the non-magnetic particles exhibit from dark-red to yellowishbrown color. Further, tbese magnetic recording media have a surfaceresistivity value as high as about 10¹³ Ω/cm², and a frictioncoefficient (as an index of the running property) as poor as about 0.33.

[0029] Further, in the case of the above-mentioned substrate using asnon-magnetic particles for non-magnetic undercoat layer, blackish brownacicular hematite particles or blackish brown acicular iron oxidehydroxide particles, the obtained non-magnetic undercoat layer can showa higher degree of blackness as compared to those using the dark redacicular hematite particles or the yellowish brown acicular iron oxidehydroxide particles, so that it becomes possible to reduce a lighttransmittance of the substrate. However, the reduction of lighttransmittance is still insufficient. In addition, the surfaceresistivity of the non-magnetic substrate is as large as about 10¹²Ω/cm², and a friction coefficient (as an index of the running property)as poor as about 0.33.

[0030] In the case of a magnetic recording medium produced by usingnon-magnetic particles for a non-magnetic undercoat layer described inEuropean Patent No. 0924690 A, due to an excellent blackness andconductivity of carbon black, the light transmittance and surfaceresistivity value of the magnetic recording medium are improved.Especially, most excellent linear absorption as an index of the lighttransmittance thereof is 2.71 μm⁻¹ and the surface resistivity valuethereof is 1.2×10⁸ Ω/cm². As shown in Comparative example describedlater, the friction coefficient as an index of the running property isas poor as about 0.32.

[0031] In the case of the non-magnetic particles described in theabove-mentioned Japanese Patent Application Laid-open (KOKAI) Nos.1-213822(1989), 1-300419(1989) and 9-297911(1997), there have been usedthe mixture of non-magnetic iron oxide particles and carbon black fineparticles in which the carbon black fine particles are added in anamount of not less than 25 parts by weight based on 100 parts by weightof the non-magnetic iron oxide particles. Therefore, due to the factthat the carbon black fine particles which show the largest degree ofblackness among various black pigments, are used in such a large amount,it is possible to obtain a non-magnetic substrate having a low lighttransmittance and a low electric resistance. However, it has beendifficult to disperse the carbon black fine particles in vehicles, sincethe carbon black fine particles are fine particles having such a smallaverage diameter as about 0.002 to about 0.05 μm, a large specificsurface area and a deteriorated solvent wettability, thereby failing toobtain a non-magnetic substrate having a smooth surface. Also, thefriction coefficient (as an index of the running property) is as poor asabout 0.31. In addition, the carbon black fine particles have a bulkdensity as low as about 0.1 g/cm³ and, therefore, the carbon black fineparticles are bulky particles, resulting in deteriorated handingproperty and workability. Further, with respect to the carbon black fineparticles, many problems concerning safety or hygiene such ascarcinogenesis have been pointed out.

[0032] Thus, as the amount of the carbon black fine particles added tothe non-magnetic substrate is increased, the light transmittance of theobtained non-magnetic substrate tends to become small. However, when thecarbon black fine particles are used in a large amount, it becomes moredifficult to disperse the particles in vehicles, resulting indeteriorated workability. Further, the use of a large amount of thecarbon black fine particles are disadvantageous in view of safety andhygiene.

[0033] In the above-mentioned Japanese Patent Application Laid-open open(KOKAI) No. 6-236542(1994), there have been described such non-magneticparticles mixture of non-magnetic iron oxide particles and carbon blackfine particles having a specific structure in which the carbon blackfine particles are used in an amount of 1 to 17.6 parts by weight basedon 100 parts by weight of the non-magnetic iron oxide particles. Byusing such specific carbon black fine particles having a highconductivity, the electric resistance of the obtained non-magneticsubstrate can be reduced even at small carbon black fine particlescontent. However, since the amount of carbon black fine particles usedis small, it is difficult to reduce a light transmittance of thenon-magnetic substrate.

[0034] As a result of the present inventors' earnest studies for solvingthe above conventional problems, it has been found that by using asnon-magnetic particles non-magnetic acicular black iron-based compositeparticles which comprise acicular hematite particles or acicular ironoxide hydroxide particles as core particles; a coating layer formed onthe surface of each core particle, at least one organosilicon compound;and a carbon black coat formed on at least a part of the surface of thecoating layer in an amount of 21 to 50 parts by weight based on 100parts by weight of the acicular hematite particles or acicular ironoxide hydroxide particles, and which have an average major axialdiameter of 0.011 to 0.35 μm and a myristic acid absorption of 0.01 to0.3 mg/m², the obtained magnetic recording medium can exhibit a smoothsurface, a lower light transmittance, a lower surface resistivity value,a low friction coefficient and an excellent running property. Thepresent invention has been attained on the basis of this finding.

SUMMARY OF THE INVENTION

[0035] An object of the present invention is to provide a magneticrecording medium exhibiting a smooth surface, a lower lighttransmittance, a lower surface resistivity value, a low frictioncoefficient and an excellent running property.

[0036] Another object of the present invention is to providenon-magnetic acicular black iron-based composite particles exhibiting anexcellent dispersibility in vehicle due to less amount of carbon blackdesorbed or fallen-off from the surface of each composite particle, amore excellent blackness, a lower volume resistivity value and awell-controlled myristic acid absorption, and a process for producingthe composite particles.

[0037] A further object of the present invention is to provide anon-magnetic substrate for magnetic recording media, having a smoothsurface and a lower surface resistively value.

[0038] To accomplish the aims, in a first aspect of the presentinvention, there is provided a magnetic recording medium comprising:

[0039] a non-magnetic base film;

[0040] a non-magnetic undercoat layer formed on the non-magnetic basefilm, comprising a binder resin and non-magnetic acicular blackiron-based composite particles; and

[0041] a magnetic coating film formed on the non-magnetic undercoatlayer, comprising a binder resin and magnetic particles,

[0042] the said non-magnetic acicular black iron-based compositeparticles having an average major axis diameter of usually 0.011 to 0.35μm, comprising:

[0043] acicular hematite particles or acicular iron oxide hydroxideparticles;

[0044] a coating layer formed on the surface of the acicular hematiteparticle or acicular iron oxide hydroxide particle, comprising at leastone organosilicon compound selected from the group consisting of:

[0045] (1) organosilane compounds obtained from an alkoxysilanecompounds, and

[0046] (2) polysiloxanes or modified polysiloxanes; and

[0047] a single carbon black coat formed on at least a part of thecoating layer comprising the organosilicon compound, in an amount of 21to 50 parts by weight based on 100 parts by weight of the acicularhematite particles or acicular iron oxide hydroxide particles.

[0048] In a second aspect of the present invention, there is provided amagnetic recording medium comprising:

[0049] a non-magnetic base film;

[0050] a non-magnetic undercoat layer formed on the non-magnetic basefilm, comprising a binder resin and non-magnetic acicular blackiron-based composite particles; and

[0051] a magnetic coating film formed on the non-magnetic undercoatlayer, comprising a binder resin and magnetic particles,

[0052] the said non-magnetic acicular black iron-based compositeparticles having an average major axis diameter of usually 0.011 to 0.35μm, comprising:

[0053] acicular hematite particles or acicular iron oxide hydroxideparticles;

[0054] a coat formed on at least a part of the surface of the acicularhematite particles or acicular iron oxide hydroxide particles,comprising at least one compound selected from the group consisting ofhydroxides of aluminum, oxides of aluminum, hydroxides of silicon andoxides of silicon;

[0055] a coating layer formed on the said coat formed on the surface ofthe acicular hematite particle or acicular iron oxide hydroxideparticle, comprising at least one organosilicon compound selected fromthe group consisting of:

[0056] (1) organosilane compounds obtained from an alkoxysilanecompounds, and

[0057] (2) polysiloxanes or modified polysiloxanes; and

[0058] a single carbon black coat formed on at least a part of thecoating layer comprising the organosilicon compound, in an amount of 21to 50 parts by weight based on 100 parts by weight of the acicularhematite particles or acicular iron oxide hydroxide particles.

[0059] In a third aspect of the present invention, there is provided anon-magnetic substrate comprising:

[0060] a non-magnetic base film; and

[0061] a non-magnetic undercoat layer formed on the non-magnetic basefilm, comprising a binder resin and non-magnetic acicular blackiron-based composite particles,

[0062] the said non-magnetic acicular black iron-based compositeparticles having an average major axis diameter of usually 0.011 to 0.35μm, comprising:

[0063] acicular hematite particles or acicular iron oxide hydroxideparticles;

[0064] a coating layer formed on the surface of the acicular hematiteparticle or acicular iron oxide hydroxide particle, comprising at leastone organosilicon compound selected from the group consisting of:

[0065] (1) organosilane compounds obtained from an alkoxysilanecompounds, and

[0066] (2) polysiloxanes or modified polysiloxanes; and

[0067] a single carbon black coat formed on at least a part of thecoating layer comprising the organosilicon compound, in an amount of 21to 50 parts by weight based on 100 parts by weight of the acicularhematite particles or acicular iron oxide hydroxide particles.

[0068] In a fourth aspect of the present invention, there is provided anon-magnetic substrate comprising:

[0069] a non-magnetic base film; and

[0070] a non-magnetic undercoat layer formed on the non-magnetic basefilm, comprising a binder resin and non-magnetic acicular blackiron-based composite particles,

[0071] the said non-magnetic acicular black iron-based compositeparticles having an average major axis diameter of usually 0.011 to 0.35μm comprising:

[0072] acicular hematite particles or acicular iron oxide hydroxideparticles;

[0073] a coat formed on at least a part of the surface of the acicularhematite particles or acicular iron oxide hydroxide particles,comprising at least one compound selected from the group consisting ofhydroxides of aluminum, oxides of aluminum, hydroxides of silicon andoxides of silicon;

[0074] a coating layer formed on the said coat formed on the surface ofthe said acicular hematite particle or acicular iron oxide hydroxideparticle, comprising at least one organosilicon compound selected fromthe group consisting of:

[0075] (1) organosilane compounds obtained from an alkoxysilanecompounds, and

[0076] (2) polysiloxanes or modified polysiloxanes; and

[0077] a single carbon black coat formed on at least a part of thecoating layer comprising the organosilicon compound, in an amount of 21to 50 parts by weight based on 100 parts by weight of the acicularhematite particles or acicular iron oxide hydroxide particles.

[0078] In a fifth aspect of the present invention, there are providednon-magnetic acicular black iron-based composite particles having anaverage major axis diameter of usually 0.011 to 0.35 μm, comprising:

[0079] acicular hematite particles or acicular iron oxide hydroxideparticles;

[0080] a coating layer formed on the surface of the acicular hematiteparticle or acicular iron oxide hydroxide particle, comprising at leastone organosilicon compound selected from the group consisting of:

[0081] (1) organosilane compounds obtained from an alkoxysilanecompounds, and

[0082] (2) polysiloxanes or modified polysiloxanes; and

[0083] a single carbon black coat formed on at least a part of thecoating layer comprising the organosilicon compound, in an amount of 21to 50 parts by weight based on 100 parts by weight of the acicularhematite particles or acicular iron oxide hydroxide particles.

[0084] In a sixth aspect of the present invention, there are providednon-magnetic acicular black iron-based composite particles having anaverage major axis diameter of usually 0.011 to 0.35 μm, comprising:

[0085] acicular hematite particles or acicular iron oxide hydroxideparticles;

[0086] a coat formed on at least a part of the surface of the acicularhematite particles or acicular iron oxide hydroxide particles,comprising at least one compound selected from the group consisting ofhydroxides of aluminum, oxides of aluminum, hydroxides of silicon andoxides of silicon;

[0087] a coating layer formed on the said coat formed on the surface ofthe acicular hematite particle or acicular iron oxide hydroxideparticle, comprising at least one organosilicon compound selected fromthe group consisting of:

[0088] (1) organosilane compounds obtained from an alkoxysilanecompounds, and

[0089] (2) polysiloxanes or modified polysiloxanes; and

[0090] a single carbon black coat formed on at least a part of thecoating layer comprising the organosilicon compound, in an amount of 21to 50 parts by weight based on 100 parts by weight of the acicularhematite particles or acicular iron oxide hydroxide particles.

DETAILED DESCRIPTION OF THE INVENTION

[0091] Firstly, a magnetic recording medium and a non-magnetic substratetherefor according to the present invention is described.

[0092] The magnetic recording medium according to the present inventioncomprises:

[0093] a non-magnetic base film;

[0094] a non-magnetic undercoat layer formed on the non-magnetic basefilm, comprising a binder resin and non-magnetic acicular blackiron-based composite particles; and

[0095] a magnetic coating film formed on the non-magnetic undercoatlayer, comprising a binder resin and magnetic particles.

[0096] The non-magnetic substrate according to the present inventioncomprises:

[0097] a non-magnetic base film; and

[0098] a non-magnetic undercoat layer formed on the non-magnetic basefilm, comprising a binder resin and the non-magnetic acicular blackiron-based composite particles.

[0099] The non-magnetic acicular black iron-based composite particlesused as non-magnetic particles contained in a non-magnetic undercoatlayer, have an average major axis diameter of 0.011 to 0.35 μm, andcomprise:

[0100] acicular hematite particles or acicular iron oxide hydroxideparticles;

[0101] a coating layer formed on the surface of the acicular hematiteparticle or acicular iron oxide hydroxide particle, comprising at leastone organosilicon compound, and

[0102] a single carbon black coat formed on at least a part of thecoating layer comprising the organosilicon compound coated, in an amountof 21 to 50 parts by weight based on 100 parts by weight of the acicularhematite particles or acicular iron oxide hydroxide particles.

[0103] The particle shape and particle size of the non-magnetic acicularblack iron-based composite particles according to the present inventionare considerably varied depending upon those of acicular hematiteparticles or acicular iron oxide hydroxide particles as core particles.Specifically, the non-magnetic acicular black iron-based compositeparticles are substantially similar in particle shape to that of thecore particles, and have a slightly larger particle size than that ofthe core particles.

[0104] The non-magnetic acicular black iron-based composite particlesaccording to the present invention have an average major axis diameterof usually 0.011 to 0.35 μm, preferably 0.018 to 0.30 μm, morepreferably 0.024 to 0.24 μm; an average minor axis diameter of usually0.006 to 0.18 μm, preferably 0.012 to 0.15 μm, more preferably 0.015 to0.12 μm; an aspect ratio (average major axis diameter/average minor axisdiameter) of usually 2:1 to 20:1, preferably 2.5:1 to 18:1, morepreferably 3:1 to 15:1; and a BET specific surface area of usually 35 to300 m²/g, preferably 38 to 250 m²/g, more preferably 40 to 230 m²/g.

[0105] When the average major axis diameter of the non-magnetic acicularblack iron-based composite particles is less than 0.011 μm, theintermolecular force between the particles may be increased due to thefineness thereof, so that it may become difficult to uniformly dispersethe particles in a vehicle. On the other hand, when the average majoraxis diameter thereof is more than 0.35 μm, since the non-magneticacicular black iron-based composite particles are coarse, the surfacesmoothness of the coating film formed using such particles may beimpaired.

[0106] Further, it is preferred that the non-magnetic acicular blackiron-based composite particles according to the present invention have ageometrical standard deviation of major axis diameter of not more than1.50. When the geometrical standard deviation of major axis diameter ismore than 1.50, since coarse particles tend to be present in thenon-magnetic acicular black iron-based composite particles, the surfacesmoothness of the coating film formed using such particles may beimpaired. With the consideration of the surface smoothness, thegeometrical standard deviation of major axis diameter of thenon-magnetic acicular black iron-based composite particles according tothe present invention is preferably not more than 1.48, more preferablynot more than 1.45. Further, in view of industrial production of thenon-magnetic acicular black iron-based composite particles, the lowerlimit of the geometrical standard deviation of major axis diameter ispreferably 1.01.

[0107] The upper limit of the blackness of the non-magnetic acicularblack iron-based composite particles according to the present inventionis usually 23.0 when represented by a L* value thereof. When the L*value as the upper limit of the blackness is more than 23.0, since thelightness of the non-magnetic acicular black iron-based compositeparticles is increased, it is difficult to reduce the lighttransmittance of the magnetic recording medium. The preferable upperlimit of the blackness thereof is 22.5.

[0108] The upper limit of the blackness of the non-magnetic acicularblack iron-based composite particles obtained by using the acicularmanganese-containing hematite particles or the acicularmanganese-containing iron oxide hydroxide particles as the coreparticles, is usually 21.5, preferably 20.5 when represented by a L*value thereof.

[0109] The lower limit of the blackness is preferably about 15 whenrepresented by a L* value.

[0110] The volume resistivity of the non-magnetic acicular blackiron-based composite particles according to the present invention ispreferably not more than 1×10⁶ Ω·cm, more preferably 1×10¹ to 5×10⁵Ω·cm, still more preferably 1 ×10¹ to 1×10⁵ Ω·cm. When the volumeresistivity is more than 1×10⁶ Ω·cm, it is difficult to sufficientlylower the surface resistivity of the magnetic recording medium.

[0111] The non-magnetic acicular black iron-based composite particlesaccording to the present invention exhibit a myristic acid absorption ofusually 0.01 to 0.3 mg/m², preferably 0.01 to 0.28 mg/m², morepreferably 0.01 to 0.26 mg/m².

[0112] When the myristic acid absorption is less than 0.01 mg/m², theamount of myristic aced absorbed onto the non-magnetic acicular blackiron-based composite particles is too small, so that it may becomedifficult to effectively control the amount of myristic acid oozed ontothe surface of the magnetic recording layer. As a result, the obtainedmagnetic recording medium such as magnetic tape may not maintain asufficiently low friction coefficient for a long period of time uponrepeated use thereof.

[0113] When the myristic acid absorption is more than 0.3 mg/m² theamount of myristic acid absorbed onto the non-magnetic acicular blackiron-based composite particles is too large, so that the amount ofmyristic acid oozed onto the surface of the magnetic recording layer isconsiderably reduced. As a result, it may become difficult to assure agood running property of the obtained magnetic recording medium.

[0114] The percentage of desorption of carbon black from thenon-magnetic acicular black iron-based composite particles according tothe present invention is usually not more than 20%, preferably not morethan 10%. When the desorption percentage of carbon black is more than20%, the desorbed carbon black tend to inhibit the composite particlesfrom being uniformly dispersed in a vehicle upon the production ofnon-magnetic coating composition.

[0115] In the non-magnetic acicular black iron-based composite particlesaccording to the present invention, the surfaces of the acicularhematite particles or the acicular iron oxide hydroxide particles as thecore particles may be preliminarily coated with at least one compoundselected from the group consisting of hydroxide of aluminum, oxides ofaluminum, hydroxides of silicon and oxides of silicon (hereinafterreferred to as “hydroxides and/or oxides of aluminum and/or silicon”),if required. In this case, the dispersibility of the obtained compositeparticles in a vehicle may become improved as compared to those havingno undercoat composed of hydroxides and/or oxides of aluminum and/orsilicon, because the percentage of desorption of carbon black from thenon-magnetic acicular black iron-based composite particles is lessened.

[0116] The amount of the hydroxides and/or oxides of aluminum and/orsilicon coat is 0.01 to 50% by weight calculated as Al, SiO₂ or a sum ofAl and SiO₂, based on the weight of the acicular hematite particles orthe acicular iron oxide hydroxide particles as the core particles.

[0117] By coating the core particle with the hydroxides and/or oxides ofaluminum and/or silicon, the percentage of desorption of carbon blackfrom the obtained non-magnetic acicular black iron-based compositeparticles of the present invention can be lessened effectively.

[0118] The non-magnetic acicular black iron-based composite particlesusing as core particles the acicular hematite particles or the aciculariron oxide hydroxide particles having the coat composed of thehydroxides and/or oxides of aluminum and/or silicon may be substantiallyidentical in a particle size, a geometrical standard deviation, a BETspecific surface area, a blackness (L* value), a volume resistivity anda myristic acid absorption, to those having no hydroxides and/or oxidesof aluminum and/or silicon coat.

[0119] By coating the core particle with the hydroxides and/or oxides ofaluminum and/silicon, the percentage of desorption of carbon black fromthe obtained non-magnetic acicular black iron-based composite particlesof the present invention is preferably not more than 10%, morepreferably not more than 5%.

[0120] The acicular hematite particles as core particles assume usuallya red color and the acicular iron oxide hydroxide particles assumeusually yellow color. In order to produce non-magnetic acicular blackiron-based composite particles having a more excellent blackness, it ispreferred to use acicular manganese-containing hematite particles oracicular manganese-containing iron oxide hydroxide particles, both whichcontain manganese in an amount of 5 to 40% by weight based on the weightof the acicular manganese-containing hematite particles or acicularmanganese-containing iron oxide hydroxide particles used as the coreparticles of the non-magnetic acicular black iron-based compositeparticles according to the present invention.

[0121] The particle shape of the acicular hematite particles or theacicular iron oxide hydroxide particles as the core particles mayinclude not only acicular shape but also spindle shape, rice ball shapeor the like.

[0122] The average major axis diameter of the acicular hematiteparticles or the acicular iron oxide hydroxide particles as the coreparticles is usually 0.01 to 0.3 μm, preferably 0.015 to 0.25 μm, morepreferably 0.02 to 0.2 μm.

[0123] If the average major axis diameter thereof exceeds 0.3 μm, sincethe average major axis diameter of the obtained non-magnetic acicularblack iron-based composite particles therefrom become coarse particles,the surface smoothness of the coating film formed using such particlesmay be impaired. On the other hand, it the average major axis diameterthereof is less than 0.01 μm the non-magnetic particles may becomeextremely fine, so that the agglomeration of the non-magnetic particlestends to occur due to the increased intermolecular force therebetween.As a result, it is difficult to uniformly coat with an organosiliconcompound and to uniformly coat with a carbon black through theorganosilicon compound.

[0124] The average minor axis diameter of the acicular hematiteparticles or the acicular iron oxide hydroxide particles as the coreparticles is usually 0.005 to 0.15 μm, preferably 0.010 to 0.125 μm,more preferably 0.012 to 0.10 μm.

[0125] If the average minor axis diameter thereof exceeds 0.15 μm, sincethe average minor axis diameter of the obtained non-magnetic acicularblack iron-based composite particles therefrom become coarse particles,the surface smoothness of the coating film formed using such particlesmay be impaired. On the other hand, if the average minor axis diameterthereof is less than 0.005 μm, the non-magnetic particles may becomeextremely fine, so that the agglomeration of the non-magnetic particlestends to occur due to the increased intermolecular force therebetween.As a result, it is difficult to uniformly coat with an organosiliconcompound and to uniformly coat with a carbon black through theorganosilicon compound.

[0126] The aspect ratio (=average major axis diameter:average minor axisdiameter, hereinafter referred to merely as “aspect ratio”) of theacicular hematite particles or the acicular iron oxide hydroxideparticles as the core particles is usually 2:1 to 20:1, preferably 2.5:1to 18:1, more preferably 3:1 to 15:1. When the aspect ratio is more than20:1, the particles may be entangled with each other, so that theagglomeration of the non-magnetic particles tends to occur due to theincreased intermolecular force therebetween. As a result, it isdifficult to uniformly coat with an organosilicon compound and touniformly coat with a carbon black through the organosilicon compound.On the other hand, when the aspect ratio is less than 2:1, it may bedifficult to obtain a coating film having a sufficient strength.

[0127] The BET specific surface area of the acicular hematite particlesor the acicular iron oxide hydroxide particles as the core particles isusually 35 to 250 m²/g, preferably 38 to 200 m²/g, more preferably 40 to180 m²/g.

[0128] If the BET specific surface area thereof is more than 250 m²/g,the increase of the intermolecular force due to the fine particles. As aresult, it may be difficult to uniformly coat with the organosiliconcompounds, and to uniformly form the carbon black coat on the coatinglayer composed of the organosilicon compounds. On the other hand, if theBET specific surface area thereof is less than 35 m²/g, the acicularhematite particles or the acicular iron oxide hydroxide particles may becoarse particles or large particles produced by sintering a particle andbetween particles, which are apt to exert a deleterious influence on thesurface smoothness of the coating film formed using such particles.

[0129] The geometrical standard deviation of the major axis diameter ofthe acicular hematite particles or the acicular iron oxide hydroxideparticles as the core particles is usually not more than 1.50. If thegeometrical standard deviation of the major axis diameter thereofexceeds 1.50, due to the coarse particles, it is difficult to uniformlycoat with an organosilicon compound and to uniformly coat with a carbonblack through the organosilicon compound. With the consideration ofuniformly coating the surfaces of the hematite particles or iron oxidehydroxide particles with the organosilicon compounds, and uniformlyforming the carbon black coat on the coating layer composed of theorganosilicon compounds, the upper limit thereof is preferably 1.48,more preferably not more than 1.45. From the point of view of industrialproductivity, the lower limit thereof is preferably 1.01.

[0130] With respect of the blackness of the acicular hematite particlesas the core particles, in case of the acicular hematite particles, thelower limit of the blackness thereof, when represented by the L* value,is usually 18, and the upper limit thereof is usually 38, preferably 36.In the case of the acicular black manganese-containing hematiteparticles, the lower limit of the blackness thereof as the coreparticles when represented by he L* value, is usually 18, and the upperlimit thereof is usually 30, preferably 28.

[0131] With respect to the blackness of the acicular iron oxidehydroxide particles as the core particles, in the case of the aciculariron oxide hydroxide particles, the lower limit of the blacknessthereof, when represented by the L* value, is usually 18, and the upperlimit thereof is usually 40, preferably 38. In the case of the acicularblack manganese-containing iron oxide hydroxide particles, the lowerlimit of the blackness thereof, when represented by the L* value, isusually 18, and the upper limit thereof is usually 32, preferably 30.

[0132] When the L* value is more than the above upper limit, theblackness of the core particles is insufficient, thereby failing toobtain the non-magnetic acicular black iron-based composite particleshaving an excellent blackness.

[0133] The volume resistivity value of the acicular hematite particlesor acicular iron oxide hydroxide particles as the core particles isusually not more than 5.0×10⁹ Ω·cm.

[0134] The myristic acid absorption of the acicular hematite particlesor acicular iron oxide hydroxide particles as the core particles isusually 0.40 to 1.0 mg/m², preferably 0.40 to 0.80 mg/m².

[0135] The surfaces of the acicular hematite particles or the aciculariron oxide hydroxide particles as the core particles may be coated withat least one compound selected from the group consisting of hydroxide ofaluminum, oxides of aluminum, hydroxides of silicon and oxides ofsilicon (hereinafter referred to as “hydroxides and/or oxides ofaluminum and/or silicon”), if required. In this case, the percentage ofdesorption of carbon black of the obtained composite particles in avehicle may become improved as compared to those having no undercoatcomposed of hydroxides or oxides of aluminum or silicon.

[0136] The amount of the hydroxides and/or oxides of aluminum and/orsilicon coat is preferably 0.01 to 50% by weight (calculated as Al,SiO₂, or a sum of Al and SiO₂) based on the total weight of the coreparticles. When the amount of the hydroxides and/or oxides of aluminumand/or silicon coat is in the range of 0.01 to 50% by weight, it ispossible to effectively reduce the carbon black desorption percentage.

[0137] The coating layer formed on the surfaces of the core particlescomprises at least one organosilicon compound selected from the groupconsisting of: (1) organosilane compounds obtained from alkoxysilanecompounds; (2) polysiloxanes; and (2′) modified polysiloxanes selectedfrom the group consisting of: (2-A) polysiloxanes modified with at leastone compound selected from the group consisting of polyethers,polyesters and epoxy compounds (hereinafter referred to merely as“modified polysiloxanes”), and (2-B) polysiloxanes whose molecularterminal is modified with at least one group selected from the groupconsisting of carboxylic acid groups, alcohol groups and a hydroxylgroup (hereinafter referred to merely as “terminal-modifiedpolysiloxanes”).

[0138] The organosilane compounds (1) can be produced from alkoxysilanecompounds represented by the formula (I):

R¹ _(a)SiX_(4−a)  (I)

[0139] wherein R¹ is C₆H₅—, (CH₃)₂CHCH₂— or n—C_(b)H_(2b—1) — (wherein bis an integer of 1 to 18); X is CH₃O— or C₂H₅O—; and a is an integer of0 to 3.

[0140] The alkoxysilane compounds may be dried or heat-treated, forexample, at a temperature of usually 40 to 200° C., preferably 60 to150° C. for usually 10 minutes to 12 hours, preferably 30 minutes to 3hours.

[0141] Specific examples of the alkoxysilane compounds may includemethyltriethoxysilane, dimethyldiethoxysilane, phenyltriethyoxysilane,diphenyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane,phenyltrimethoxysilane, diphenyldimethoxysilane,isobutyltrimethoxysilane, decyltrimethoxysilane or the like. Among thesealkoxysilane compounds, in view of the desorption percentage and theadhering effect of carbon black, methyltriethoxysilane,phenyltriethyoxysilane, methyltrimethoxysilane, dimethyldimethoxysilaneand isobutyltrimethoxysilane are preferred, and methyltriethoxysilaneand methyltrimethoxysilane are more preferred.

[0142] As the polysiloxanes (2), there may be used those compoundsrepresented by the formula (II):

[0143] wherein R² is H— or CH₃—, and d is an integer of 15 to 450.

[0144] Among these polysiloxanes, in view of the desorption percentageand the adhering effect of carbon black, polysiloxanes having methylhydrogen siloxane units are preferred.

[0145] As the modified polysiloxanes (2-A), there may be used:

[0146] (a1) polysiloxanes modified with polyethers represented by theformula (III):

[0147] wherein R³ is —(—CH₂—)_(h)—; R⁴ is —(—CH₂—)_(i)—CH₃; R⁵ is —OH,—COOH, —CH═CH₂, —C(CH₃)═CH₂ or —(—CH₂—)_(j)—CH₃; R⁶ is —(—CH₂—)_(k)—CH₃;g and h are an integer of 1 to 15; i, j and k are an integer of 0 to 15;e is an integer of 1 to 50; and f is an integer of 1 to 300;

[0148] (a2) polysiloxanes modified with polyesters represented by theformula (IV):

[0149] wherein R⁷, R⁸ and R⁹ are —(—CH₂—)_(q)— and may be the same ordifferent; R¹⁰ is —OH, —COOH, —CH═CH₂, —C(CH₃)═CH₂ or —(—CH₂—)_(r)—CH₃;R¹¹ is —(—CH₂—)_(s)—CH₃; n and q are an integer of 1 to 15; r and s arean integer of 0 to 15; e′ is an integer of 1 to 50; and f′ is an integerof 1 to 300;

[0150] (a3) polysiloxanes modified with epoxy compounds represented bythe formula (V):

[0151] wherein R¹² is —(—CH₂—)_(v)—; v is an integer of 1 to 15; t is aninteger of 1 to 50; and u is an integer of 1 to 300; or a mixturethereof.

[0152] Among these modified polysiloxanes (2-A), in view of thedesorption percentage and the adhering effect of carbon black, thepolysiloxanes modified with the polyethers represented by the formula(III), are preferred.

[0153] As the terminal-modified polysiloxanes (2-B), there may be usedchose represented by the formula (VI):

[0154] wherein and are R¹³ and R¹⁴ are —OH, R¹⁶OH or R¹⁷COOH and may bethe same or different; R¹⁵ is —CH₃ or —C₆H₅; R¹⁶ and R¹⁷ are—(—CH₂—)_(y)—; y is an integer of 1 to 15; w is an integer of 1 to 200;and x is an integer of 0 to 100.

[0155] Among these terminal-modified polysiloxanes, in view of thedesorption percentage and the adhering effect of carbon black, thepolysiloxanes whose terminals are modified with carboxylic acid groupsare preferred.

[0156] The amount of the coating layer composed of the organosiliconcompounds is usually 0.02 to 5.0% by weight, preferably 0.03 to 4.0% byweight, more preferably 0.05 to 3.0% by weight (calculated as Si) basedon the weight of the acicular hematite particles or the acicular ironoxide hydroxide particles coated with the organosilicon compounds.

[0157] When the amount of the coating layer composed of theorganosilicon compounds is less than 0.02% by weight, it may becomedifficult to adhere a carbon black on the coating layer. On the otherhand, even when the coating amount of the organosilicon compounds ismore than 5.0% by weight, a sufficient amount of carbon black coat canbe formed on the surface of the coating layer.

[0158] The a carbon black coat constituting by a single carbon blackcoat is formed on at least a part of the coating layer composed of theorganosilicon compounds.

[0159] The total amount of carbon black adhered is 21 to 50 parts byweight based on 100 parts by weight of the core particles.

[0160] When the amount of carbon black coated is less than 21 parts byweight, it is difficult to reduce the myristic acid absorption to notmore than 0.3 mg/m². On the contrary, when the amount of carbon blackcoated is more than 50 parts by weight, the myristic acid absorption ofthe obtained composite particles is as low as less than 0.01 mg/m². As aresult, it becomes difficult to effectively control the amount ofmyristic acid oozed onto the surface of the magnetic recording layer.

[0161] The thickness of carbon black coat formed is preferably not morethan 0.06 μm, more preferably not more than 0.05 μm, still morepreferably not more than 0.04 μm. The lower limit thereof is morepreferably 0.0001 μm.

[0162] Next, the non-magnetic substrate according to the presentinvention is described.

[0163] The non-magnetic substrate according to the present invention isconstituted by a non-magnetic base film and a non-magnetic undercoatlayer formed on the non-magnetic base film, comprising a binder resinand the non-magnetic acicular black iron-based composite particles.

[0164] As the non-magnetic base film, the following materials which areat present generally used for the production of a magnetic recordingmedium are usable as a raw material: a synthetic resin such aspolyethylene terephthalate, polyethylene, polypropylene, polycarbonate,polyethylene naphthalate, polyamide, polyamideimide and polyimide; foiland plate of a metal such as aluminum and stainless steel; and variouskinds of paper. The thickness of the non-magnetic base film variesdepending upon the material, but it is usually about 1.0 to 300 μm,preferably 2.0 to 200 μm.

[0165] In the case of a magnetic disc, polyethylene terephthalate isusually used as the non-magnetic base film. The thickness thereof isusually 50 to 300 μm, preferably 60 to 200 μm. In the case of a magnetictape, when polyethylene terephthalate is used as the base film, thethickness thereof is usually 3 to 100 μm, preferably 4 to 20 μm. Whenpolyethylene naphthalate is used, the thickness thereof is usually 3 to50 μm, preferably 4 to 20 μm. When polyamide is used, the thicknessthereof is usually 2 to 10 μm, preferably 3 to 7 μm.

[0166] The non-magnetic undercoat layer of the present inventionpreferably has a film thickness of 0.2 to 10.0 μm. When the thickness ofthe non-magnetic undercoat layer is less than 0.2 μm, it is difficult toimprove the surface roughness of the non-magnetic substrate, and thestiffness of the non-magnetic undercoat layer tends to be insufficient.In the consideration of reduction in thickness of the obtained magneticrecording medium and stiffness of the coating film, the thickness of thenon-magnetic undercoat layer is more preferably in the range of 0.5 to5.0 μm.

[0167] As the binder resin used in the present invention, the followingresins which are at present generally used or the production of amagnetic recording medium are usable: vinyl chloride-vinyl acetatecopolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acidcopolymer, urethane elastomer, butadiene-acrylonitrile copolymer,polyvinyl butyral, cellulose derivative such as nitrocellulose,polyester resin, synthetic rubber resin such as polybutadiene, epoxyresin, polyamide resin, polyisocyanate, electron radiation curing acrylurethane resin and mixtures thereon.

[0168] Each of these resin binders may contain a functional group suchas —OH, —COOH, —SO₃M, —OPO₂M₂ and —NH₂, wherein M represents H, Na or K.With the consideration of the dispersibility of the particles, a binderresin containing a functional group —COOH or —SO₃M is preferable.

[0169] The mixing ratio of the non-magnetic acicular black iron-basedcomposite particles with the binder resin is usually 5 to 2000 parts byweight, preferably 100 to 1000 parts by weight based on 100 parts byweight of the binder resin.

[0170] When the amount of the non-magnetic acicular black iron-basedcomposite particles blended is less than 5 parts by weight, the amountof the non-magnetic acicular black iron-based composite particlescontained in the non-magnetic coating composition is too small. As aresult, there may not be formed a coating film in which the non-magneticacicular black iron-based composite particles are continuouslydispersed, and the surface smoothness of the coating film and thestiffness of the non-magnetic substrate may become insufficient. Whenthe amount of the non-magnetic acicular black iron-based compositeparticles blended is more than 2,000 parts by weight, the amount of thenon-magnetic acicular black iron-based composite particles becomes toolarge as compared to that of the binder resin, thereby failing tosufficiently disperse the non-magnetic acicular black iron-basedcomposite particles in the non-magnetic coating composition. As aresult, it may be difficult to form a coating film having a sufficientlysmooth surface. Further, the non-magnetic acicular black iron-basedcomposite particles may not be sufficiently bound together by the binderresin, so that the obtained coating film becomes brittle.

[0171] A lubricant, a polishing agent, an antistatic agent, etc. whichare generally used for the production of a magnetic recording medium maybe added to the non-magnetic undercoat layer in amount of usually 0.1 to50 parts by weight based on 100 parts by weight of the binder resin.

[0172] The non-magnetic substrate according to the present invention hasthe following properties.

[0173] The non-magnetic substrate according to the present invention hasa gloss (of the coating film) of usually 170 to 280%; a surfaceroughness Ra (of the coating film) of usually 2.0 to 12.0 nm; a Young'smodulus (relative value to a commercially available video tape: and AVT-120 produced by Victor Company of Japan, Limited) of usually 117 to150; a linear adsorption coefficient (of the coating film) of usually1.50 to 5.00 μm₁; and a surface resistivity of 1×10³ to 1×10¹¹ Ω/cm².

[0174] In case of using the non-magnetic acicular black iron-basedcomposite particles as non-magnetic particles, in which theorganosilicon compound is formed on the surface of the acicular hematiteparticle or acicular iron oxide hydroxide particle, the non-magneticsubstrate according to the present invention has a gloss (of the coatingfilm) of usually 170 to 280%, preferably 175 to 280%, more preferably180 to 280%; a surface roughness Ra (of the coating film) of usually 2.0to 12.0 nm, preferably 2.0 to 11.5 nm, more preferably 2.0 to 11.0 nm; aYoung's modulus (relative value to a commercially available video tape:and AV T-120 produced by Victor Company of Japan, Limited) of usually117 to 150, preferably 119 to 150, more preferably 121 to 150; a linearadsorption coefficient (of the coating film) of usually 1.50 to 5.00μm³¹ ¹, preferably 1.55 to 5.00 μm³¹ ¹; and a surface resistivity of1×10³ to 1×10¹¹ Ω/cm², preferably 1×10³ to 5×10¹⁰ Ω/cm², more preferably1×10³ to 1×10¹⁰ Ω/cm².

[0175] In case of using the non-magnetic acicular black iron-basedcomposite particles as non-magnetic particles, in which theorganosilicon compound is formed on the hydroxides and/or oxides ofaluminum and/or silicon coat which are formed on the surface of theacicular hematite particle or acicular iron oxide hydroxide particle,the non-magnetic substrate according to the present invention has agloss (of the coating film) of usually 175 to 280%, preferably 180 to280%, more preferably 185 to 280%; a surface roughness Ra (of thecoating film) of usually 2.0 to 11.5 nm, preferably 2.0 to 11.0 nm, morepreferably 2.0 to 10.5 nm; a Young's modulus (relative value to acommercially available video tape: and AV T-120 produced by VictorCompany of Japan, Limited) of usually 118 to 150, preferably 120 to 150,more preferably 122 to 150; a linear adsorption coefficient (of thecoating film) of usually 1.50 to 5.00 μm³¹ ¹, preferably 1.55 to 5.00μm³¹ ¹; and a surface resistivity of 1×10³ to 1×10¹¹ Ω/cm², preferably1×10³ to 5×10¹⁰ Ω/cm², more preferably 1×10³ to 1×10¹⁰ Ω/cm².

[0176] Next, the magnetic recording medium according to the presentinvention is described.

[0177] The magnetic recording medium according to the present inventionis constituted by the non-magnetic substrate and a magnetic coating filmformed on the non-magnetic substrate, comprising a binder resin andmagnetic particles.

[0178] As the magnetic particles used in the present invention, magneticparticles containing iron as a main component are usable, and there maybe exemplified magnetic iron oxide particles such as maghemiteparticles, magnetite particles and berthollide compound particles whichare an intermediate oxide between maghemite and magnetite; particlesobtained by incorporating any one or more different kinds of elementsother than Fe, such as Co, Al, Ni, P, Zn, Si, B or the like in the saidmagnetic iron oxide particles; Co modified particles obtained bymodifying the said magnetic iron oxide particles with cobalt; magneticacicular metal particles containing iron as a main component, which maycontain elements other than Fe at least one selected from the groupconsisting of Co, Al, Ni, P, Si, Zn, B, Nd, La and Y, including magneticacicular iron-based alloy particles; magnetoplumbite-type ferriteparticles such as plate-like ferrite particles containing Ba, Sr orBa-Sr; plate-like magnetoplumbite-type ferrite particles obtained byincorporating other metals such as Co, Ni, Zn, Mg, Mn, Ti, Sn, Zr, Nb,Cu, Mo or the like as a coercive force-reducingg agent in the plate-likemagnetoplumbite-type ferrite particles; or the like.

[0179] With the consideration of the short-wavelength recording and thehigh-density recording, Co-coated magnetic iron oxide particles,magnetic acicular metal particles containing iron as a main componentand magnetic acicular iron-based alloy particles containing elementsother than Fe at least one selected from the group consisting of Co, Al,Ni, P, Si, Zn, B, Nd, La, Y or the like are preferable.

[0180] The magnetic acicular iron-based alloy particles comprising (i)iron and Al; (ii) iron, Co and Al, (iii) iron, Al and at least onerare-earth metal such as Nd, La and Y, or (iv) iron, Co, Al and at leastone rare-earth metal such as Nd, La and Y is even more preferable fromthe point of the durability of the magnetic recording medium. Further,the magnetic acicular iron-based alloy particles comprising iron, Al andat least one rare-earth metal such as Nd, La and Y is most preferable.

[0181] More specifically. the magnetic acicular iron-based alloyparticles may be exemplified as follows.

[0182] 1) Magnetic acicular iron-based alloy particles comprises iron;and cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% byweight, more preferably 3 to 30% by weight (calculated as Co) based onthe weight of the magnetic acicular iron-based alloy particles.

[0183] 2) Magnetic acicular iron-based alloy particles comprises iron;and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% byweight (calculated as Al) based on the weight of the magnetic aciculariron-based alloy particles.

[0184] 3) Magnetic acicular iron-based alloy particles comprises iron;cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% byweight, more preferably 3 to 30% by weight (calculated as Co) based onthe weight of the magnetic acicular iron-based alloy particles; andaluminum of usually 0.05 on 10% by weight, preferably 0.1 to 7% byweight (calculated as Al) based on the weight of the magnetic aciculariron-based alloy particles.

[0185] 4) Magnetic acicular iron-based alloy particles comprises iron;cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% byweight, more preferably 3 to 30% by weight (calculated as Co) based onthe weight of the magnetic acicular iron-based alloy particles; and atleast one selected from the group consisting of Nd, La and Y of usually0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as thecorresponding element) based on the weight of the magnetic aciculariron-based alloy particles.

[0186] 5) Magnetic acicular iron-based alloy particles comprises iron,aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% byweight (calculated as Al) based on the weight of the magnetic aciculariron-based alloy particles; and at least one selected from the groupconsisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably0.1 to 7% by weight (calculated as the corresponding element) based onthe weight of the magnetic acicular iron-based alloy particles.

[0187] 6) Magnetic acicular iron-based alloy particles comprises iron;cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% byweight, more preferably 3 to 30% by weight (calculated as Co) based onthe weight of the magnetic acicular iron-based alloy particles; aluminumof usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight(calculated as Al) based on the weight of the magnetic aciculariron-based alloy particles; and at least one selected from the groupconsisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably0.1 to 7% by weight (calculated as the corresponding element) based orthe weight of the magnetic acicular iron-based alloy particles.

[0188] 7) Magnetic acicular iron-based alloy particles comprises iron;cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% byweight, more preferably 3 to 30% by weight (calculated as Co) based onthe weight of the magnetic acicular iron-based alloy particles; and atleast one selected from the group consisting of Ni, P, Si, Zn, Ti, Cuand B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight(calculated as the corresponding element) based on the weight of themagnetic acicular iron-based alloy particles.

[0189] 8) Magnetic acicular iron-based alloy particles comprises iron;aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% byweight (calculated as Al) based on the weight of the magnetic aciculariron-based alloy particles; and at least one selected from the groupconsisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% byweight, preferably 0.1 to 7% by weight (calculated as the correspondingelement) based on the weight of the magnetic acicular iron-based alloyparticles.

[0190] 9) Magnetic acicular iron-based alloy particles comprises iron;cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% byweight, more preferably 3 to 30% by weight (calculated as Co) based onthe weight of the magnetic acicular iron-based alloy particles; aluminumof usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight(calculated as Al) based or the weight of the magnetic aciculariron-based alloy particles; and at least one selected from the groupconsisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% byweight, preferably 0.1 to 7% by weight (calculated as the correspondingelement) based on the weight of the magnetic acicular iron-based alloyparticles.

[0191] 10) Magnetic acicular iron-based alloy particles comprises iron;cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% byweight, more preferably 3 to 30% by weight (calculated as Co) based onthe weight of the magnetic acicular iron-based alloy particles; at leastone selected from the group consisting of Nd, La and Y of usually 0.05to 10% by weight, preferably 0.1 to 7% by weight (calculated as hecorresponding element) based on the weight of the magnetic aciculariron-based alloy particles; and at least one selected from the groupconsisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% byweight, preferably 0.1 to 7% by weight (calculated as the correspondingelement) based on the weight of the magnetic acicular iron-based alloyparticles.

[0192] 11) Magnetic acicular iron-based alloy particles comprises iron;aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% byweight (calculated as Al) based on the weight of the magnetic aciculariron-based alloy particles; at least one selected from the groupconsisting of Nd, La and Y of ordinarily 0.05 to 10% by weight,preferably 0.1 to 7% by weight (calculated as the corresponding element)based on the weight of the magnetic acicular iron-based alloy particles;and at least one selected from the group consisting of Ni, P, Si, Zn,Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% byweight (calculated as the corresponding element) based on the weight ofthe magnetic acicular iron-based alloy particles.

[0193] 12) Magnetic acicular iron-based alloy particles comprises iron;cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% byweight, more preferably 3 to 30% by weight (calculated as Co) based onthe weight of the magnetic acicular iron-based alloy particles; aluminumof usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight(calculated as Al) based on the weight of the magnetic aciculariron-based alloy particles; at least one selected from the groupconsisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably0.1 to 7% by weight (calculated as the corresponding element) based onthe weight of the magnetic acicular iron-based alloy particles; and atleast one selected from the group consisting of Ni, P, Si, Zn, Ti, Cuand B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight(calculated as the corresponding element) based on the weight of themagnetic acicular iron-based alloy particles.

[0194] The iron content in the particles is the balance, and ispreferably 50 to 99% by weight, more preferably 60 to 95% by weight(calculated as Fe) based on the weight of the magnetic acicular metalparticles containing iron as a main component or the magnetic aciculariron-based alloy particles.

[0195] It is preferred that the shape of the magnetic particles isacicular, cubic or plate-like. The acicular shape may include not onlyneedle-shape but also spindle-shape, rice ball-shape, or the like.

[0196] In the case that the shape of the magnetic particles is acicular,the magnetic particles used in the present invention have an averagemajor as diameter of usually 0.01 to 0.50 μm, preferably 0.03 to 0.30μm, an average minor axis diameter of usually 0.0007 to 0.17 μm,preferably 0.003 to 0.10 μm.

[0197] In the case that the shape of the magnetic particles isplate-like, the magnetic particles used in the present invention have anaverage particle size of usually 0.01 to 0.50 μm, preferably 0.03 to0.30 μm, an average thickness of usually 0.0007 to 0.17 μm, preferably0.003 to 0.10 μm.

[0198] The geometrical standard deviation of the major axis diameter ofthe magnetic particles used in the present invention is preferably notmore than 2.5, more preferably 2.3. From the point of view of industrialproductivity, the lower limit of the geometrical standard deviation ofthe major axis diameter is preferably 1.01.

[0199] The BET specific surface area of the magnetic particles used inthe present invention is usually 35 to 100 m²/g, preferably 38 to 90m²/g, more preferably 40 to 80 m²/g.

[0200] In the case that the shape of the magnetic particles is acicular,the magnetic particles have an aspect ratio of usually not less than3:1, preferably and not less than 5:1. The upper limit of the aspectratio is usually 15:1, preferably 10:1 with the consideration of thedispersibility in the vehicle.

[0201] In the case that the shape of the magnetic particles isplate-like, the magnetic particles have a plate ratio (an averageparticle size/average thickness) of usually not less than 2:1,preferably and not less than 3:1. The upper limit of the plate ratio isusually 20:1, preferably 15:1 with the consideration of thedispersibility in the vehicle.

[0202] As to the magnetic properties of the magnetic particles used inthe present invention, the coercive force is usually 19.9 to 318.3 kA/m(250 to 4000 Oe), and the saturation magnetization is usually 40 to 170Am²/kg (40 to 170 emu/g).

[0203] As to the magnetic properties of the magnetic iron oxideparticles or Co-coated magnetic iron oxide particles used in the presentinvention, the coercive force is usually 19.9 to 135.3 kA/m (250 to 1700Oe), preferably 23.9 to 135.3 kA/m (300 to 1700 Oe), and the saturationmagnetization is usually 60 to 90 Am²/kg (60 to 90 emu/g), preferably 65to 90 Am²/kg (65 to 90 emu/g).

[0204] As to the magnetic properties of the magnetic acicular metalparticles containing iron as a main component or magnetic aciculariron-based alloy particles used in the present invention, the coerciveforce is usually 63.7 to 278.5 kA/m (800 to 3500 Oe), preferably 71.6 to278.5 kA/m (900 to 3500 Oe), and the saturation magnetization is usually90 to 170 Am²/kg (90 to 170 emu/g) preferably 100 to 170 Am²/kg (100 to170 emu/g).

[0205] As to the magnetic properties of the magnetoplumbite-type ferriteparticles used in the present invention, the coercive force is usually39.8 to 318.3 kA/m (500 to 4000 Oe), preferably 51.7 to 318.3 kA/m (650to 4000 Oe), and the saturation magnetization is usually 40 to 70 Am²/kg(40 to 70 emu/g), preferably 45 to 70 Am²/kg (45 to 70 emu/g).

[0206] As the binder resin for the magnetic recording layer, the samebinder resin as that used for the production of the non-magneticundercoat layer is usable.

[0207] The thickness or the magnetic recording layer obtained byapplying the magnetic coating composition on the surface of thenon-magnetic undercoat layer and dried, is usually in the range of 0.01to 5.0 μm. The thickness is less than 0.01 μm, uniform coating may bedifficult, so that unfavorable phenomenon such as unevenness on thecoating surface is observed. On the other hand, when the thicknessexceeds 5.0 μm, it may be difficult to obtain desired electromagneticperformance due to an influence of diamagnetism. The preferablethickness is in the range of 0.05 to 1.0 μm.

[0208] The mixing ratio of the magnetic acicular metal particlescontaining iron as a main component with the binder resin in themagnetic recording layer is usually 200 to 2000 parts by weight,preferably 300 to 1500 parts by weight based on 100 parts by weight ofthe binder resin.

[0209] A lubricant, a polishing agent, an antistatic agent, etc., whichare generally used or the production of a magnetic recording medium maybe added to the magnetic recording layer in an amount of usually 0.1 to50 parts by weight based on 100 parts by weight of the binder resins.

[0210] The magnetic recording medium according to the present inventionhas the following properties.

[0211] The magnetic recording medium according to the present inventionhas a coercive force of usually 19.9 to 318.3 kA/m (250 to 4000 Oe); asquareness (residual magnetic flux density Br/saturation magnetic fluxdensity Bm) of usually 0.85 to 0.95; a gloss (of the coating film) ofusually 130 to 300%; a surface roughness Ra (of the coating film) ofusually not more than 12.0 nm; a Young's modulus (relative value to acommercially available video tape: AV T-120 produced by Victor Companyof Japan, Limited) of usually 122 to 160; a linear adsorptioncoefficient (of the coating film) of usually 1.90 to 10.0 μm³¹ ¹; asurface resistivity of not more than 1×10⁹ Ω/cm²; and a frictioncoefficient of usually 0.05 to 0.30.

[0212] In case where the non-magnetic acicular black iron-basedcomposite particles in which the organosilicon compound is formed on thesurface of the acicular hematite particle or acicular iron oxidehydroxide particle, are used as non-magnetic magnetic particles, themagnetic recording medium according to the present invention has acoercive force of usually 19.9 to 318.3 kA/m (250 to 4000 Oe),preferably 23.9 to 318.3 kA/m (300 to 4000 Oe); a squareness (residualmagnetic flux density Br/saturation magnetic flux density Bm) of usually0.85 to 0.95, preferably 0.36 to 0.95; a gloss (of the coating film) ofusually 130 to 300%, preferably 140 to 300%; a surface roughness Ra (ofthe coating film) of usually not more than 12.0 nm, preferably 2.0 to11.0 nm, more preferably 2.0 to 10.0 nm; a Young's modulus (relativevalue to a commercially available video tape: AV T-120 produced byVictor Company of Japan, Limited) of usually 122 to 160, preferably 124to 160; a linear adsorption coefficient (of the coating film) of usually1.90 to 10.00 μm³¹ ¹, preferably 2.00 to 10.00 μm³¹ ¹; a surfaceresistivity of not more than 1×10⁹ Ω/cm², preferably not more than7.5×10 Ω/cm², more preferably not more than 5×10⁸ Ω/cm²; and a frictioncoefficient of usually 0.05 to 0.30, preferably 0.05 to 0.28, morepreferably 0.05 to 0.26.

[0213] In case where the non-magnetic acicular black iron-basedcomposite particles in which the organosilicon compound is formed on thehydroxides and/or oxides of aluminum and/or silicon coat formed on thesurface of the acicular hematite particle or acicular iron oxidehydroxide particle, are used as non-magnetic particles, the magneticrecording medium according to the present invention has a coercive forceof usually 19.9 to 318.3 kA/m (250 to 4000 Oe), preferably 23.9 to 318.3kA/m (300 to 4000 Oe); a squareness (residual magnetic flux densityBr/saturation magnetic flux density Bm) of usually 0.85 to 0.95,preferably 0.86 to 0.95; a gloss (of the coating film) of usually 135 to300%, preferably 145 to 300%; a surface roughness Ra (of the coatingfilm) of usually not more than 11.5 nm, preferably 2.0 to 10.5 nm, morepreferably 2.0 to 9.5 nm; a Young's modulus (relative value to acommercially available video tape: AV T-120 produced by Victor Companyof Japan, Limited) of usually 124 to 160, preferably 126 to 160; alinear adsorption coefficient (of the coating film) of usually 1.90 to10.00 μm³¹ ¹, preferably 2.00 to 10.00 μm³¹ ¹; a surface resistivity ofnot more than 1×10⁹ Ω/cm², preferably not more than 7.5×10⁸ Ω/cm², morepreferably not more than 5×10⁸ Ω/cm²; and a friction coefficient ofusually 0.05 to 0.30, preferably 0.05 to 0.28, more preferably 0.05 to0.26.

[0214] In case where magnetic acicular metal particles containing ironas a main component or magnetic acicular iron-based alloy particles areused as the magnetic particles, and the non-magnetic acicular blackiron-based composite particles in which the organosilicon compound isformed on the surface of the acicular hematite particle or acicular ironoxide cm²hydroxide particle, are used as non-magnetic particles, themagnetic recording medium according to the present invention has acoercive force of usually 63.7 to 278.5 kA/m (800 to 3500 Oe),preferably 71.6 to 278.5 kA/m (900 to 3500 Oe); a squareness (residualmagnetic flux density Br/saturation magnetic flux density Bm) of usually0.85 to 0.95, preferably 0.86 to 0.95; a gloss (of the coating film) ofusually 185 to 300%, preferably 190 to 300%; a surface roughness Ra (ofthe coating film) of usually not more than 9.5 nm, preferably 2.0 to 9.0nm, more preferably 2.0 to 8.5 nm; a Young's modulus (relative value toa commercially available video tape: AV T-120 produced by Victor Companyof Japan, Limnted) of usually 122 to 160, preferably 124 to 160; alinear adsorption coefficient (of the coating film) of usually 1.90 to10.00 μm³¹ ¹, preferably 2.00 to 10.00 μm³¹ ¹; a surface resistivity ofnot more than 1 ×10⁹ Ω/cm², preferably not more than 7.5×10⁸ Ω/cm², morepreferably not more than 5×10⁸ Ω/cm², and a friction coefficient ofusually 0.05 to 0.30, preferably 0.05 to 0.28, more preferably 0.05 to0.26.

[0215] In case where magnetic acicular metal particles containing ironas a main component or magnetic acicular iron-based alloy particles areused as the magnetic particles, and the non-magnetic acicular blackiron-based composite particles in which the organosilicon compound isformed on the hydroxides and/or oxides of aluminum and/or silicon coatformed on the surface of the acicular hematite particle or acicular ironoxide hydroxide particle. are used as non-magnetic particles, themagnetic recording medium according to the present invention has acoercive force of usually 63.7 to 278.5 kA/m (800 to 3500 Oe),preferably 71.6 to 278.5 kA/m (900 to 3500 Oe); a squareness (residualmagnetic flux density Br/saturation magnetic flux density Bm) of usually0.85 to 0.95, preferably 0.86 to 0.95, a gloss (of the coating film) ofusually 190 to 300%, preferably 195 to 300%; a surface roughness Ra (ofthe coating film) of usually not more than 9.0 nm, preferably 2.0 to 8.5nm, more preferably 2.0 to 8.0 nm; a Young's modulus (relative value toa commercially available video tape; AV T-120 produced by Victor Companyof Japan, Limited) of usually 124 to 160, preferably 126 to 160; alinear adsorption coefficient (of the coating film) of usually 1.90 to10.00 μm³¹ ¹, preferably 2.00 to 10.00 μm³¹ ¹; a surface resistivity ofnot more than 1×10⁹ Ω/cm², preferably not more than 7.5×10⁸ Ω/cm², morepreferably not more than 5×10⁸ Ω/cm²; and a friction coefficient ofusually 0.05 to 0.30, preferably 0.05 to 0.28, more preferably 0.05 to0.26.

[0216] Next, the process for producing the non-magnetic acicular blackiron-based composite particles according to the present invention, isdescribed.

[0217] The acicular goethite particles as the core particles may beproduced by a so-called wet process, i.e., by passing anoxygen-containing gas such as air through a suspension containing eitherferrous hydroxide colloid, iron carbonate or iron-containingprecipitates obtained by reacting a ferrous salt aqueous solution withalkali hydroxide, alkali carbonate or both of alkali hydroxide andalkali carbonate, and then after filtering-out and washing with water.The acicular hematite particles as the core particles may be produced byheat-treating the obtained acicular goethite particles at a temperatureof 250 to 850° C. in an oxygen-containing gas such as air.

[0218] The acicular manganese-containing hematite particles as the coreparticles may be produced by heat-treating acicular goethite particlescontaining manganese in an amount of 8 to 150 atomic % based on whole Fewhich are obtained by the below-mentioned method, at a temperature of250 to 850° C. in an oxygen-containing gas such as air.

[0219] The acicular manganese-containing goethite particles as the coreparticles may be produced by conducting the above-mentioned wet processfor producing the acicular goethite particles, in the presence ofmanganese, thereby forming acicular goethite particles containingmanganese in an amount of 8 to 150 atomic % based on whole Fe.

[0220] Alternatively, elements other than Fe such as Ni, Zn, P and Si,which are generally added in order to enhance various properties of theparticles such as the major axis diameter, the minor axis diameter andthe aspect ratio, may be added during the reaction system for producingthe acicular goethite particles.

[0221] Especially, in advance of the heat-dehydration of the aciculargoethite particles for producing the acicular hematite particles, it ispreferred that the surfaces of the acicular goethite particles arecoated with sintering preventive, as is well known in the art. Thecoating treatment of the sintering preventive is composed of the stepsof: adding the sintering preventive to an aqueous suspension containingthe acicular goethite particles, mixing and stirring the resultantsuspension, filtering out the particles, washing the particles withwater, and drying the particles.

[0222] As the sintering preventive, known sintering preventives areusable. For example, phosphorus compounds such as sodiumhexametaphosphate, polyphospholic acid and orthophosphoric acid, siliconcompounds such as #3 water glass, sodium orthosilicate, sodiummetasilicate and colloidal silica, boron compounds such as boric acid,aluminum compounds including aluminum salts such as aluminum acetate,aluminum sulfate, aluminum chloride and aluminum nitrate, alkalialuminate such as sodium aluminate, alumina sol and aluminum hydroxide,and titanium compounds such as titanyl sulfate may be exemplified,orthophosphoric acid, colloidal silica, boric acid and aluminum acetateare preferable.

[0223] The coating of the acicular hematite particles or the aciculariron oxide hydroxide particles with the alkoxysilane compounds, thepolysiloxanes, the modified polysiloxanes or the terminal-modifiedpolysiloxanes, may be conducted (i) by mechanically mixing and stirringthe acicular hematite particles or the acicular iron oxide hydroxideparticles together with the alkoxysilane compounds, the polysiloxanes,the modified polysiloxanes or the terminal-modified polysiloxanes; or(ii) by mechanically mixing and stirring both the components togetherwhile spraying the alkoxysilane compounds, the polysiloxanes, themodified polysiloxanes or the terminal-modified polysiloxanes onto theacicular hematite particles or the acicular iron oxide hydroxideparticles. In these cases, substantially whole amount of thealkoxysilane compounds, the polysiloxanes, the modified polysiloxanes orthe terminal-modified polysiloxanes added can be applied onto thesurfaces of the acicular hematite particles or the acicular iron oxidehydroxide particles.

[0224] In addition, by conducting the above mixing or stirring treatmentof the core particles together with the alkoxysilane compounds, at leasta part of the alkoxysilane compounds coated on the core particles may bechanged to the organosilane compounds.

[0225] In order to uniformly coat the surfaces of the acicular hematiteparticles or the acicular iron oxide hydroxide particles with thealkoxysilane compounds, the polysiloxanes, the modified polysiloxanes orthe terminal-modified polysiloxanes, it is preferred that the acicularhematite particles or the acicular iron oxide hydroxide particles arepreliminarily diaggregated by using a pulverizer.

[0226] As apparatuses used for (1) mixing and stirring the coreparticles with alkoxysilane compounds, the polysiloxanes, the modifiedpolysiloxanes or the terminal-modified polysiloxanes; (2) mixing andstirring the carbon black fine particles with the particlessurface-coated with alkoxysilane compounds, the polysiloxanes, themodified polysiloxanes or the terminal-modified polysiloxanes; (3)mixing and stirring the adhesive with the particles having a carbonblack coat formed onto the surface-coating composed of alkoxysilanecompounds, the polysiloxanes, the modified polysiloxanes or theterminal-modified polysiloxanes (hereinafter referred to as “compositeparticles”); and (4) mixing and stirring the carbon black fine particleswith the composite particles coated with the adhesive, there may bepreferably used apparatus capable of applying a shearing force to alayer or the particles to be treated, more preferably those capable ofconducting shearing, spatula-stroking and compression at the same time,for example, wheel-type kneader, ball-type kneader, blade-type kneader,roll-type kneader or the like. Among these apparatuses, the wheel-typekneader is more effective for the practice of the present invention.

[0227] Specific examples of the wheel-type kneaders may include an edgerunner (equal to a mix muller, a Simpson mill or a sand mill), amulti-mull, a Stotz mill, a wet pan mill, a Conner mill, a ring muller,or the like. Among them, an edge runner, a multi-mull, a Stotz mill, awet pan mill and a ring muller are preferred, and an edge runner is morepreferred.

[0228] Specific examples of the ball-type kneaders may include avibrating mill or the like. Specific examples of the blade-type kneadersmay include a Henschel mixer, a planetary mixer, a Nawter mixer or thelike. Specific examples of the roll-type kneaders may include anextruder or the like.

[0229] In order to coat the surfaces of the core particles with thealkoxysilane compounds, the polysiloxanes, the modified polysiloxanes orthe terminal-modified polysiloxanes as uniformly as possible, theconditions of the above mixing or stirring treatment may beappropriately controlled such that the linear load is usually 19.6 to1960 N/cm (2 to 200 Kg/cm), preferably 98 to 1470 N/cm (10 to 150Kg/cm), more preferably 147 to 980 N/cm (15 to 100 Kg/cm); and thetreating time is usually 5 to 120 minutes, preferably 10 to 90 minutes.It is preferred to appropriately adjust the stirring speed in the rangeof usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10to 800 rpm.

[0230] The amount of the alkoxysilane compounds, the polysiloxanes, themodified polysiloxanes or the terminal-modified polysiloxanes added, ispreferably 0.15 to 45 parts by weight based on 100 parts by weight ofthe acicular hematite particles or the acicular iron oxide hydroxideparticles.

[0231] When the amount of the alkoxysilane compounds, the polysiloxanes,the modified polysiloxanes or the terminal-modified polysiloxanes addedis less than 0.15 part by weight, it may become difficult to form thecarbon black coaton the coating layer. On the other hand, when theamount of the alkoxysilane compounds, the polysiloxanes, the modifiedpolysiloxanes or the terminal-modified polysiloxanes added is more than45 parts by weight, it is meaningless to add such unnecessarily largeamount thereof because a sufficient amount of the carbon black coat canbe formed on the surface of the coating layer. Next, the carbon blackfine particles are added to the acicular hematite particles or theacicular iron oxide hydroxide particles coated with the alkoxysilanecompounds, the polysiloxanes, the modified polysiloxanes or theterminal-modified polysiloxanes, and the resultant mixture is mixed andstirred to form a carbon black coat on at least a part of the coatinglayer composed of the alkoxysilane compounds, the polysiloxanes, themodified polysiloxanes or the terminal-modified polysiloxanes added.

[0232] As the carbon black fine particles used in the present invention,there may be exemplified commercially available carbon blacks such asfurnace black, channel black or the like. Specific examples of thecommercially available carbon blacks usable in the present invention,may include #3050, #3150, #3250, #3750, #3950, MA100, MA7, #1000,#2400B, #30, MA77, MA8, #650, MA11, #50, #52, #45, #2200B, MA600, etc.(tradename, produced by MITSUBISHI CHEMICAL CORP.), SEAST 9H, SEAST 7H,SEAST 6, SEAST 3H, SEAST 300, SEAST FM, etc. (tradename, produced byTOKAI CARBON CO., LTD.), Raven 1250, Raven 860 ULTRA, Raven 1000, Raven1190 ULTRA, etc. (tradename, produced by COLOMBIAN CHEMICALS COMPANY),Ketchen black EC, Ketchen black EC600JD, etc. (tradename, produced byKETCHEN BLACK INTERNATIONAL CO., LTD.), BLACK PEARLS-L, BLACK PEARLS1000, BLACX PEARLS 4630, VULCAN XC72, REGAL 660, REGAL 400, etc.(tradename, produced by CABOTT SPECIALTY CHEMICALS INK CO., LTD.), orthe like.

[0233] In the consideration of the effect of reducing the myristic acidabsorpti on, the use of carbon black having a pH value of not more than8.0 is preferred. Specific examples of the commercially available carbonblacks usable in the present invention, may include #3050, ·3150, #3250,#3750, #3950, MA100, MA7, #1000, #2400B, #30, MA77, MA8, #650, MA11,#50, #52, #45, #2200B, MA600, etc. (tradename, produced by MITSUBISHICHEMICAL CORP.), SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, SEASTFM, etc. (tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250,Raven 860 ULTRA, Raven 1000, Raven 1190 ULTRA. etc. (tradename, producedby COLOMBIAN CHEMICALS COMPANY), BLACK PEARLS-L, BLACK PEARLS 1000,REGAL 660, REGAL 400, etc. (tradename, produced by CABOTT SPECIALTYCHEMICALS INK CO., LTD.).

[0234] Further, in the consideration of uniform adhesion onto theorganosilane compound coating layer, polysiloxane coating layer ordimethylpolysiloxane coating layer, the use of carbon black fineparticles having a DBP oil absorption of not more than 180 ml/100 g ismore preferred. Specific examples of the commercially available carbonblacks usable in the present invention, may include #3050, #3150, #3250,MA100, MA7, #1000, #2400B, #30, MA77, MA8, #650, MA11, #50, #52, #45,#2200B, MA600, etc. (tradename, produced by MITSUBISHI CHEMICAL CORP.),SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, SEAST FM, etc.(tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250, Raven 860ULTRA, Raven 1000, Raven 1190 ULTRA, etc. (tradename, produced byCOLOMBIAN CHEMICALS COMPANY), BLACK PEARLS-L, BLACK PEARLS 1000, REGAL660, REGAL 400, etc. (tradename, produced by CABOTT SPECIALTY CHEMICALSINK CO., LTD.).

[0235] The average particle size of the carbon black fine particles usedis usually 0.002 to 0.05 μm, preferably 0.002 to 0.035 μm. When theaverage particle size of the carbon black fine particles used is lessthan 0.002 μm, the carbon black fine particles used are too fine to bewell handled.

[0236] On the other hand, when the average particle size thereof is morethan 0.05 μm, since the particle size of the carbon black fine particlesused is much larger, it is necessary to apply a larger mechanical shearforce for forming the uniform carbon black coat on the coating layercomposed of the organosilicon compounds, thereby rendering the coatingprocess industrially disadvantageous.

[0237] The carbon black fine particles are added to the acicularhematite particles or the acicular iron oxide hydroxide particles coatedwith the alkoxysilane compounds, the polysiloxanes, the modifiedpolysiloxanes or the terminal-modified polysiloxanes in an amount of 21to 50 parts by weight based on 100 parts by weight of the acicularhematite particles or acicular iron oxide hydroxide particles, and theresultant mixture is mixed and stirred to form a carbon black coat(single carbon black coat) on at least a part of the coating layercomposed of the alkoxysilane compounds, the polysiloxanes, the modifiedpolysiloxanes or the terminal-modified polysiloxanes.

[0238] By varying an adding method of the carbon black fine particles,mixing and stirring conditions, it is possible to form the single carbonblack coat on the coating layer composed of the organosilicon compounds,thereby rendering the coating process industrially disadvantageous.

[0239] As the adding method, a lump addition method, a continuousaddition method, a divided addition method may be exemplified. When alarge amount of the carbon black fine particles is added, it ispreferred to conduct the continuous addition method and the dividedaddition method.

[0240] In case of continuously adding the carbon black fine particles,the carbon black fine particles may be added slowly and little bylittle, especially for a period of 5 minutes to 24 hours, preferably 5minutes to 20 hours. The mixing and stirring step under the ollowingconditions can be conducted.

[0241] In case of dividedly adding the carbon black fine particles, theadding step of the carbon black fine particles of 5 to 25 parts byweight based on 100 parts by weight of the acicular hematite particlesor acicular iron oxide hydroxide particles. The mixing and stirring stepunder the following conditions can be repeated until the added amount ofthe carbon black fine particles reaches a predetermined amount thereof.

[0242] The mixing and stirring conditions may be appropriately selectedso as to form a uniform carbon black coat on the coating layer composedof the organosilicon compounds, and may be controlled such that thelinear load is usually 19.6 to 1,960 N/cm (2 to 200 kg/cm), preferably98 to 1,470 N/cm (10 to 150 kg/cm), more preferably 147 to 980 N/cm (15to 100 kg/cm); the treating time is usually 5 minutes to 24 hours,preferably 10 minutes to 20 hours; and the stirring speed is usually 2to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.

[0243] After the carbon black coat is formed on the surface of thecoating layer, the resultant composite particles may be dried orheat-treated, for example, at a temperature of usually 40 to 200° C.,preferably 60 to 150° C. for usually 10 minutes to 12 hours, preferably30 minutes to 3 hours.

[0244] The alkoxysilane used to coat the core particles in the thusobtained non-magnetic acicular black iron-based composite particles isfinally converted into the organosilane compound through the abovemixing and stirring step and drying or heat-treating step.

[0245] At least a part of the surface of the acicular hematite particlesor the acicular iron oxide hydroxide particles may be coated with atleast one compound selected from the group consisting of hydroxides ofaluminum, oxides of aluminum, hydroxides of silicon and oxides ofsilicon, if required, in advance of mixing and stirring with thealkoxysilane compounds, the polysiloxanes, the modified polysiloxanes orthe terminal-modified polysiloxanes.

[0246] The coat of the hydroxides and/or oxides of aluminum and/orsilicon may be conducted by adding an aluminum compound, a siliconcompound or both the compounds to a water suspension in which theacicular hematite particles or the acicular iron oxide hydroxideparticles are dispersed, followed by mixing and stirring, and furtheradjusting the pH of the suspension, if required, thereby coating thesurfaces of the acicular hematite particles or the acicular iron oxidehydroxide particles with at least one compound selected from the groupconsisting of hydroxides of aluminum, oxides of aluminum, hydroxides ofsilicon and oxides of silicon. The thus obtained particles coated withthe hydroxides and/or oxides of aluminum and/or silicon are thenfiltered out, washed with water, dried and pulverized. Further, theparticles coated with the hydroxides and/or oxides of aluminum and/orsilicon may be subjected to post-treatments such as deaeration treatmentand compaction treatment, if required.

[0247] As the aluminum compounds, there may be exemplified aluminumsalts such as aluminum acetate, aluminum sulfate, aluminum chloride oraluminum nitrate, alkali aluminates such as sodium aluminate or thelike.

[0248] The amount of the aluminum compound added is 0.01 to 50% byweight (calculated as Al) based on the weight of the acicular hematiteparticles or the acicular iron oxide hydroxide particles. When theamount of the aluminum compound added is less than 0.01% by weight, itmay be difficult to sufficiently coat the surfaces of the acicularhematite particles or the acicular iron oxide hydroxide particles withhydroxides or oxides of aluminum, thereby failing to achieve theimprovement of lessening the percentage of desorption of carbon blacktherefrom. On the other hand, when the amount of the aluminum compoundadded is more than 50% by weight, the coating effect is saturated and,therefore, it is meaningless to add such an excess amount of thealuminum compound.

[0249] As the silicon compounds, there may be exemplified #3 waterglass, sodium orthosilicate, sodium metasilicate, colloidal silica orthe like.

[0250] The amount of the silicon compound added is 0.01 to 50% by weight(calculated as SiO₂) based on the weight of the acicular hematiteparticles or the acicular iron oxide hydroxide particles. When theamount of the silicon compound added is less than 0.01% by weight, itmay be difficult to sufficiently coat the surfaces of the acicularhematite particles or the acicular iron oxide hydroxide particles withhydroxides or oxides of silicon, thereby failing to achieve theimprovement of lessening the percentage of desorption or carbon blacktherefrom. On the other hand, when the amount of the silicon compoundadded is more than 50% by weight, the coating effect is saturated and,therefore, it is meaningless to add such an excess amount of the siliconcompound.

[0251] In the case where both the aluminum and silicon compounds areused in combination for the coating, the total amount of the aluminumand silicon compounds added is preferably 0.01 to 50% by weight(calculated as a sum of Al and SiO₂) based on the weight of the acicularhematite particles or the acicular iron oxide hydroxide particles.

[0252] The non-magnetic substrate according to the present invention isproduced by applying a non-magnetic coating composition which containsthe non-magnetic acicular black iron-based composite particles, a binderresin and a solvent, to the surface of the non-magnetic base film,followed by drying, thereby forming the non-magnetic undercoat.

[0253] As the solvents, there may be used methyl ethyl ketone, toluene,cyclohexanone, methyl isobutyl ketone, tetrahydrofuran, a mixture ofthese solvents or the like.

[0254] The total amount of the solvent used is 50 to 1,000 parts byweight based on 100 parts by weight of the non-magnetic acicular blackiron-based composite particles. When the amount of the solvent used isless than 50 parts by weight, the viscosity of the non-magnetic coatingcomposition prepared therefrom may become too high, thereby making itdifficult to apply the non-magnetic coating composition. On the otherhand, when the amount of the solvent used is more than 1,000 parts byweight, the amount of the solvent volatilized during the formation ofthe coating film may become too large, thereby rendering the coatingprocess industrially disadvantageous.

[0255] The magnetic recording medium according to the present inventioncan be produced by applying a magnetic coating composition containingthe magnetic particles, a binder resin and a solvent, on thenon-magnetic undercoat layer, followed by drying, to form a magneticrecording layer thereon.

[0256] As the solvents, there may be used methyl ethyl ketone, toluene,cyclohexanone, methyl isobutyl ketone, tetrahydrofuran, a mixture ofthese solvents or the like.

[0257] The total amount of the solvent used is 65 to 1,000 parts byweight based on 100 parts by weight of the magnetic particles. When theamount of the solvent used is less than 65 parts by weight, theviscosity of the magnetic coating composition prepared therefrom becomestoo high, thereby making it difficult to apply the magnetic coatingcomposition. On the other hand, when the amount of the solvent used ismore than 1,000 parts by weight, the amount of the solvent volatilizedduring the formation of the coating film becomes too large, therebyrendering the coating process industrially disadvantageous.

[0258] A point of the present invention lies in such a fact that in thecase where the carbon black is strongly bonded onto the surface of eachcore particle in an amount as large as 21 to 50 parts by weight based on100 parts by weight of the core particles, it is possible to obtainacicular black composite particles having a myristic acid absorption of0.01 to 0.3 mg/m².

[0259] The reason why the myristic acid absorption of the non-magneticmagnetic acicular black iron-based composite particles according to thepresent invention can be reduced, is considered as follows, though notclearly known. That is, as shown in Comparative Examples hereunder, themyristic acid absorption of the non-magnetic particles cannot be reducedto not more than 0.3 mg/m² in any of the cases where the amount of thecarbon black adhered is not less than 21 parts by weight and the carbonblack desorption percentage is not less than 20%, and where the carbonblack desorption percentage is less than 20% and the amount of thecarbon black adhered is less than 21 parts by weight. Due to this fact,it is considered that by forming the dense carbon black coat having anappropriate uniform thickness on the surface of each core particle, itis possible to effectively inhibit bonding between many hydroxy groupsexisting on the surface of each core particle and carboxyl groups ofmyristic acid having a high affinity to the hydroxy groups.

[0260] The magnetic recording medium of the present invention which isproduced by using the non-magnetic acicular black iron-based compositeparticles of the present invention as non-magnetic particles fornon-magnetic undercoat layer thereof, exhibits a low frictioncoefficient.

[0261] The reason why the friction coefficient of the magnetic recordingmedium according to the present invention can be reduced, is consideredas follows. That is, since the myristic acid absorption of thenon-magnetic particles incorporated into the non-magnetic undercoatlayer in a large amount, is limited to the specific range, anappropriately controlled amount of the myristic acid can be oozed ontothe surface of the magnetic recording layer for a long period of time,thereby enabling the myristc acid to effectively exhibit its function aslubricant.

[0262] In accordance with the present invention, due to the fact thatthe non-magnetic acicular black iron-based composite particles asnon-magnetic particles have an excellent dispersibility, and show anexcellent blackness and a volume resistivity, and the myristic acidabsorption of the non-magnetic acicular black iron-based compositeparticles is limited to the specific range, it s possible to produce thenon-magnetic undercoat layer showing an excellent surface smoothness, anexcellent blackness and a lower surface resistivity, and anappropriately controlled amount of the myristic acid can be oozed ontothe surface of the magnetic recording layer. The non-magnetic acicularblack iron-based composite particles according to the present inventionare suitably used as non-magnetic particles contained in thenon-magnetic undercoat layer of the magnetic recording medium suitablefor high-density recording.

[0263] By using such non-magnetic acicular black iron-based compositeparticles used as non-magnetic particles contained in a non-magneticundercoat layer, the magnetic recording medium according to the presentinvention shows an excellent surface smoothness, a lower lighttransmittance and a lower surface resistivity, and further has a lowfriction coefficient and an excellent running durability. The magneticrecording medium to the present invention is suitable for high-densityrecording.

EXAMPLES

[0264] The present invention is described in more detail by Examples andComparative Examples, but the Examples are only illustrative and,therefore, not intended to limit the scope of the present invention.

[0265] Various properties were evaluated by the following methods.

[0266] (1) The average major axis diameter, average minor axis diameterand average particle size of acicular hematite particles, acicular ironoxide hydroxide particles, carbon black fine particles, non-magneticacicular black iron-based composite particles and magnetic particleswere respectively expressed by average values (measured in apredetermined direction) of about 350 particles which were sampled froma micrograph obtained by magnifying an original electron micrograph(×30,000) by four times in each of the longitudinal and transversedirections.

[0267] (2) The aspect ratio of the particles was expressed by a ratio ofaverage major axis diameter to minor axis diameter thereof.

[0268] (3) The geometrical standard deviation of particle sizes wasexpressed by values obtained by the following method. That is, theparticle sizes were measured from the above-magnified photograph. Theactual particle sizes and the number of the particles were obtained fromthe calculation on the basis of the measured values. On a logarithmicnormal probability paper, the particle sizes were plotted at regularintervals on the abscissa-axis and the accumulative number of particlesbelonging to each interval of the particle sizes were plotted bypercentage on the ordinate-axis by a statistical technique. The particlesizes corresponding to the number of particles of 50% and 84.13%,respectively were read from the graph, and the geometrical standarddeviation was measured from the following formula:

[0269] Geometrical standard deviation=

[0270] {particle size corresponding to 84.13% under integrationsieve}/{particle size (geometrical average diameter) corresponding to50% under integration sieve}

[0271] The more the geometrical standard deviation nears 1.0, the moreexcellent the particle size distribution of the particles.

[0272] (4) The specific surface area was expressed by values measured bya BET method.

[0273] (5) The amounts of Mn, Al and Si which were present withinacicular hematite particles and acicular iron oxide hydroxide particlesor on the surfaces thereof, the amount of Si contained in organosiliconcompounds and the amount of Si contained in dimetylpolysiloxanes usedfor adhering the carbon black, were measured by a fluorescent X-rayspectroscopy device 3063M (manufactured by RIGAKU DENKI KOGYO CO., LTD.)according to JIS K0119 “General rule of fluorescent X-ray analysis”.

[0274] (6) The amount of carbon black coat formed on the non-magneticacicular black iron-based composite particles was measured by “HoribaMetal, Carbon and Sulfur Analyzer EMIA-2200 Model” (manufactured byHORIBA SEISAKUSHO CO., LTD.).

[0275] (7) The thickness or carbon black coat formed on the surfaces ofthe black iron-based composite particles is expressed by the value whichwas obtained by first measuring an average thickness of carbon blackcoat formed onto the surfaces of the particles on a photograph(×5,000,000) obtained by magnifying (ten times) a micrograph (×500,000)produced at an accelerating voltage of 200 kV using a transmission-typeelectron microscope (JEM-2010, manufactured by JAPAN ELECTRON Co.,Ltd.), and then calculating an actual thickness of carbon black coatformed from the measured average thickness.

[0276] (8) The blackness of acicular hematite particles, acicular ironoxide hydroxide particles and non-magnetic acicular black iron-basedcomposite particles, were measured by the following method. That is, 0.5g of sample particles and 1.5 ml of castor oil were intimately kneadedtogether by a Hoover's muller to form a paste. 4.5 g of clear lacquerwas added to the obtained paste and was intimately mixed to form apaint. The pain was applied on a cast-coated paper by using a 6-milapplicator to produce a coating film piece (having a film thickness ofabout 30 μm). The thus obtained coating film piece was measuredaccording to JIS Z 8729 by a multi-light source spectrographiccolorimeter MSC-IS-2D (manufactured by SUGA SHIKENKI CO., LTD.) todetermine a L* value of colorimetric indices thereof.

[0277] Here, the L* value represents a lightness, and the smaller the L*value, the more excellent the blackness.

[0278] (9) The volume resistivity or the acicular hematite particles,the acicular iron oxide hydroxide particles and the non-magneticacicular black iron-based composite particles were measured as follows.First, 0.5 g of the respective particles were weighed, andpressure-molded into a cylindrical shape at 1.372×10⁷ Pa (140 Kg/cm²)using a KBr tablet machine (manufactured by SIMAZU SEISAKUSHO CO.,LTD.), thereby producing a cylindrical sample to be measured.

[0279] The thus-produced sample was then exposed to an atmosphere keptat a temperature of 25° C. and a relative humidity of 60%, for not lessthan 12 hours. Thereafter, the sample was fixed between stainless steelelectrodes, and a voltage of 15V was applied to the sample using aWheatstone bridge (TYPE2768, manufactured by YOKOGAWA-HOKUSHIN DENKICo., LTD.), thereby measuring a resistance value R (Ω) of the sample.

[0280] Next, an upper surface area A (cm²) and a thickness t₀ (cm) ofthe cylindrical sample were measured, and the respective measured valueswere substituted for A and t₀ of the following formula to obtain thevolume resistivity (Ω·cm) of the sample.

Volume resistivity (Ω·cm)=R×(A/t ₀)

[0281] (10) The myristic acid absorption was measured by the followingmethod. The lower the myristic acid absorption, the more the fatty acidbecomes to ooze on the surface of the magnetic recording layer and thelower the friction coefficient thereof.

[0282] 100 g of 1.5 mmφ glass beads, 9 g of particles to be measured and45 ml of a tetrahydrofuran solution containing myristic acid in anamount enough to form one layer thereof on each particle, were chargedinto a 140-ml glass bottle, and then mixed and dispersed together for 60minutes using a paint shaker.

[0283] Next, the thus obtained dispersion was taken out, charged into a50-ml precipitation tube and centrifuged at 10,000 rpm for 15 minutes,thereby separating a solvent portion from a solid portion. The amount(concentration) of myristic acid contained in the solvent portion wasdetermined by a gravimetric method. By subtracting the measured valuefrom an amount of myristic acid initially charged, the amount ofmyristic acid contained in the solid portion was obtained as a myristicacid absorption (mg/m²) of the particles to be measured.

[0284] (11) The desorption percentage (%) of carbon black desorbed fromthe non-magnetic acicular black iron-based composite particles wasmeasured by the following method.

[0285] That is, 3 g of the non-magnetic acicular black iron-basedcomposite particles and 40 ml of ethanol were placed in a 50-mlprecipitation pipe and then was subjected to ultrasonic dispersion for20 minutes. Thereafter, the obtained dispersion was allowed to stand for120 minutes, and separated the carbon black desorbed from thenon-magnetic acicular black iron-based composite particles on the basisof the difference in specific gravity therebetween. Next, the thusseparated non-magnetic acicular black iron-based composite particleswere mixed again with 40 ml of ethanol, and the obtained mixture wasfurther subjected to ultrasonic dispersion for 20 minutes. Thereafter,the obtained dispersion was allowed to stand for 120 minutes, therebyseparating the non-magnetic acicular black iron-based compositeparticles and carbon black desorbed, from each other. The thus separatednon-magnetic acicular black iron-based composite particles were dried at100° C. for one hour, and then the carbon content thereof was measuredby the “Horiba Metal, Carbon and Sulfur Analyzer EMIA-2200 Model”(manufactured by HORIBA SEISAKUSHO CO., LTD.). The desorption percentage(%) was calculated according to the following formula:

Desorption percentage (%)={(W _(a) −W _(e))/W _(a)}×100

[0286] wherein W_(a) represents an amount of carbon black initiallyadhered on the non-magnetic acicular black iron-based compositeparticles; and W_(e) represents an amount of carbon black which stillremains on the non-magnetic acicular black iron-based compositeparticles after the above desorption test.

[0287] The closer to zero the desorption percentage (%), the smaller theamount of carbon black desorbed from the non-magnetic acicular blackiron-based composite particles.

[0288] (12) The viscosity of the coating composition was obtained bymeasuring the viscosity of the coating composition at 25° C. at a shearrate D of 1.92 sec⁻¹ by using “E type viscometer EMD-R” (manufactured byTOKYO KEIKI, CO., LTD.).

[0289] (13) The gloss of the surface of the coating film of each of thenon-magnetic undercoat layer and the magnetic recording layer wasmeasured at an angle of incidence of 45° by “glossmeter UGV-5D”(manufactured by SUGA SHIKENKI, CO., LTD.).

[0290] (14) The surface roughness Ra is expressed by the center-lineaverage roughness of the surface of the coating film by using“Surfcom-575A” (manufactured by TOKYO SEIMITSU CO., LTD.).

[0291] (15) The strength of the coating film was expressed the Young'smodulus obtained by “Autograph” (produced by SHIMAZU SEISAKUSHO Co.,Ltd.). The Young's modulus was expressed by the ratio of the Young'smodulus of the coating film to that of a commercially available videotape “AV T-120” (produce by VICTOR COMPANY OF JAPAN, LIMITED). Thehigher the relative value, the more the strength of the coating film isfavorable.

[0292] (16) The magnetic properties of the magnetic particles andmagnetic recording medium were measured under an external magnetic fieldof 795.8 kA/m (10 kOe) by “Vibration Sample Magnetometer VSM-3S-15(manufactured by TOEI KOGYO, CO., LTD.)”.

[0293] (17) The light transmittance is expressed by the linearadsorption coefficient measured by using “UV-Vis RecordingSpectrophotometer UV-2100” (manufactured by SHIMAZU SEISAKUSHO, Co.Ltd.). The linear adsorption coefficient is defined by the followingformula, and the larger the value, the more the transmittance of lightbecome difficult:

[0294] Linear adsorption coefficient (μm³¹ ¹)={1 n (1/t)}/FT wherein trepresents a light transmittance (−) at λ=900 nm, and FT representsthickness (μm) of the coating composition of the film used for themeasurement.

[0295] As a blank for measuring the linear adsorption coefficient, incase of the non-magnetic substrate composed of the base film and thenon-magnetic undercoat layer, the same non-magnetic base film was used,and in case of the magnetic recording medium composed of the base film,the non-magnetic undercoat layer and the magnetic recording layer, thesame non-magnetic base film was used.

[0296] (18) The surface resistivity value of a coating film was measuredas follows. The coating film to be measured was exposed to an atmospherekept at a temperature of 25° C. and a relative humidity of 60%, for notless than 12 hours. Thereafter, the coating film was slit into 6 mmwidth, and then placed on metal electrodes each having a width of 6.5 mmsuch that a coating surface thereof was contacted with the metalelectrodes. Two 170-gram weights were fitted to opposite ends of thecoating film so as to bring the coating film into close contact with themetal electrodes. Then, a D.C. voltage of 500V was applied between themetal electrodes, thereby measuring a surface resistivity value of thecoating film.

[0297] (19) The friction coefficient of the magnetic recording medium isexpressed by the value obtained from the ratio of a friction forcemeasured between the surface of magnetic tape and metal surface(aluminum mirror surface) using a tensile tester “TENSILON”(manufactured by SHIMADZU SEISAKUSHO Co., Ltd.), to a load applied.

[0298] (20) The thickness of each of the base film, the non-magneticundercoat layer and the magnetic recording layer constituting themagnetic recording medium was measured in the following manner by using“Digital Electronic Micrometer R351C” (manufactured by ANRITSU CORP.)

[0299] The thickness (A) of a base film was first measured. Similarly,the thickness (B) (B=the sum of the thicknesses of the base film and thenon-magnetic undercoat layer) of a non-magnetic substrate obtained byforming a non-magnetic undercoat layer on the base film was measured.Furthermore, the thickness (C) (C=the sum of the thicknesses of the basefilm, the non-magnetic undercoat layer and the magnetic recording layer)of a magnetic recording medium obtained by forming a magnetic recordinglayer on the non-magnetic substrate was measured. The thickness of thenon-magnetic undercoat layer is expressed by (B)-(A), and the thicknessof the magnetic recording layer is expressed by (C)-(B).

Example 1

[0300] <Production of Non-magnetic Acicular Black Iron-based CompositeParticles>

[0301] 20 kg of acicular hematite particles (average major axisdiameter: 0.143 μm; average minor axis diameter: 0.0210 μm; aspectratio: 6.8:1; geometrical standard deviation of major axis diameter:1.38; BET specific surface area value: 55.3 m²/g; blackness (L* value):28.3; volume resistivity: 2.3×10⁸ Ω·cm and myristic acid absorption:0.41 mg/m²), were diaggregated in 150 liters of pure water using astirrer, and further passed through a TK pipeline homomixer(manufactured by TOKUSHU KIKA KOGYO CO., LTD.) three times, to obtain aslurry containing the acicular hematite particles.

[0302] Successively, the obtained slurry was passed through atransverse-type sand grinder (tradename “MIGHTY MILL MHG-1.5 L”,manufactured by INOUE SEISAKUSHO Co, LTD.) five times at anaxis-rotating speed of 2,000 rpm, thereby obtaining a slurry in whichthe acicular hematite particles were dispersed.

[0303] The acicular hematite particles in the obtained slurry, whichremain on a sieve of 325 meshes (mesh size: 44 μm) was 0%. The slurrywas filtered and washed with water, thereby obtaining a wet cakecomposed of the acicular manganese-containing hematite particles. Theobtained wet cake composed of the acicular hematite particles was driedat 120° C. 11.0 kg or the dried particles were then charged into an edgerunner “MPUV-2 Model” (tradename, manufactured by MATSUMOTO CHUZOTEKKOSHO CO., LTD.), and mixed and stirred at 294 N/cm (30 Kg/cm) for 30minutes, thereby lightly diaggregating the particles.

[0304] 220 g of methyltriethoxysilane (tradename: “TSL8123”, produced byGE TOSHIBA SILICONE CO., LTD.) (2.0 parts by weight based on 100 partsby weight of the acicular hematite particles), was mixed and dilutedwith 200 ml of ethanol to obtain a solution of methyltriethoxysilane.The methyltriethoxysilane solution was added to the thus diaggregatedacicular hematite particles while operating the edge runner. Theacicular hematite particles were continuously mixed and stirred at alinear load of 588 N/cm (60 Kg/cm ) and a stirring speed of 22 rpm or 30minutes.

[0305] Next, carbon black fine particles D (particle shape: granularshape; average particle size: 0.022 μm; geometrical standard deviationof particle sizes: 1.78; BET specific surface area value: 133.5 m²/g;blackness (L* value) 14.6; pH value: 3.4; and DBP oil absorption: 84ml/100 g) of 25.0 parts by weight based on 100 parts by weight of theacicular hematite particles, were added to the acicular hematiteparticles coated with methyltriethoxysilane for 150 minutes whileoperating the edge runner. Further, the particles were continuouslymixed and stirred at a linear load of 588 N/cm (60 Kg/cm) and a stirringspeed of 22 rpm for 30 minutes to form a carbon black coat on thecoating layer composed of methyltriethoxysilane, thereby obtainingcomposite particles.

[0306] In order to determine the coating amount of methyltriethoxysilaneand the amount of carbon black adhered, a part of the thus obtainedcomposite particles was sampled, and heat-treated at 105° C. for 60minutes using a drier. As a result, it was confirmed that the coatingamount of methyltriethoxysilane was 0.29% by weight (calculated as Si)(equivalent to 2.0 parts by weight based on 100 parts by weight of theacicular hematite particles), and the amount of carbon black adhered was19.92% by weight (equivalent to 25 parts by weight based on 100 parts byweight of the acicular hematite particles). Further, as a result of theobservation of electron micrograph, it was confirmed that almost a wholeamount of carbon black added was adhered onto the coating layer of anorganosilane compound produced from the methyltriethoxysilane.

[0307] The obtained non-magnetic acicular black iron-based compositeparticles had an average major axis diameter of 0.145 μm, an averageminor axis diameter of 0.0212 μm and an aspect ratio of 6.8:1 as shownin the electron photograph. In addition, the non-magnetic acicular blackiron-based composite particles showed a geometrical standard deviationof major axis diameter of 1.38, a BET specific surface area value of56.3 m²/g, a blackness (L* value) of 17.8, a volume resistivity of6.2×10¹ Ω·cm, myristic acid absorption of 0.21 mg/m², and a desorptionpercentage of carbon black of 6.6%. The thickness of the carbon blackcoat formed was 0.0026 μm. Since no carbon black were recognized on theelectron photograph, it was confirmed that a whole amount of the carbonblack used contributed to the formation of the carbon black coat.

Example 2

[0308] <Production of Non-magnetic Substrate: Formation of Non-magneticUndercoat Layer on Base Film>

[0309] 12 g of the non-magnetic acicular black iron-based compositeparticles obtained in Example 1 were mixed with a binder resin solution(30% by weight of vinyl chloride-vinyl acetate copolymer resin having asodium sulfonate group and 70% by weight of cyclohexanone) andcyclohexanone, and each of the obtained mixtures (solid content: 72% byweight) was kneaded by a plast-mill for 30 minutes.

[0310] Each of the thus-obtained kneaded material was charged into a 140ml-glass bottle together with 95 g of 1.5 mmφ glass beads, a binderresin solution (30% by weight of polyurethane resin having a sodiumsulfonate group and 70% by weight of a solvent (methyl ethyl ketone:toluene=1: 1)), cyclohexanone, methyl ethyl ketone and toluene, and theobtained mixture was mixed and dispersed by a paint shaker for 6 hours.Thereafter, the lubricant was added to the resultant mixture, and theobtained mixture was mixed and dispersed by a paint shaker for 15minutes to obtain a non-magnetic coating composition.

[0311] The thus-obtained non-magnetic coating composition containing thenon-magnetic acicular black iron-based composite particles was asfollows: Non-magnetic acicular black iron-based   100 parts by weightcomposite particles Vinyl chloride-vinyl acetate   10 parts by weightcopolymer resin having a sodium sulfonate group Polyurethane resinhaving a   10 parts by weight sodium sulfonate group Lubricant (myristicacid: butyl  2.0 parts by weight stearate = 1:2) Cyclohexanone  56.9parts by weight Methylethyl ketone 142.3 parts by weight Toluene  85.4parts by weight

[0312] The viscosity of the obtained non-magnetic coating compositionwas 374 cP.

[0313] The non-magnetic coating composition obtained was applied to apolyethylene terephthalate film of 12 μm thick to a thickness of 55 μmby an applicator, and the coating film was then dried, thereby forming anon-magnetic undercoat layer. The thickness of the non-magneticundercoat layer was 3.5 μm.

[0314] The thus obtained non-magnetic undercoat layer had a gloss of196%, and a surface roughness Ra of 6.6 nm. The Young's modulus(relative value) thereof was 123. The linear adsorption coefficient (ofthe coating film) thereof was 3.44 μm³¹ ¹; and the surface resistivitythereof was 4.3×10⁶ Ω/cm².

Example 3

[0315] <Production of Magnetic Recording Medium: Formation of MagneticRecording Layer>

[0316] 12 g of magnetic acicular metal particles containing iron as amain component (average major axis diameter: 0.131 μm, average minoraxis diameter: 0.0180 μm, aspect ratio: 7.3:1, BET specific surface areavalue: 58.0 m²/g, geometrical standard deviation of major axis diameter:1.40, coercive force: 156.9 kA/m (1972 Oe), saturation magnetization:135.0 Am²/kg (135.0 emu/g), Al content: 2.56% by weight and Co content:5.85% by weight), 1.2 g of a polishing agent (AKP-50: trade name,produced by SUMITOMO CHEMICAL CO., LTD.), 0.12 g of carbon black(#3250B, trade name, produced by MITSUBISHI CHEMICAL CORP.), a binderresin solution (30% by weight of vinyl chloride-vinyl acetate copolymerresin having a sodium sulfonate group and 70% by weight ofcyclohexanone) and cyclohexanone were mixed to obtain a mixture (solidcontent 78% by weight). The mixture was further kneaded by a plast-millfor 30 minutes to obtain a kneaded material.

[0317] The thus-obtained kneaded material was charged into a 140ml-glass bottle together with 95 g of 1.5 mmφ glass beads, a binderresin solution (30% by weight of polyurethane resin having a sodiumsulfonate group and 70% by weight of a solvent (methyl ethyl ketone:toluene 1:1)), cyclohexanone, methyl ethyl ketone and toluene, and themixture was mixed and dispersed by a paint shaker for 6 hours. Then, thelubricant and hardening agent were added to the mixture, and theresultant mixture was mixed and dispersed by a paint shaker for 15minutes.

[0318] The thus-obtained magnetic coating composition was as follows:Magnetic acicular metal   100 parts by weight particles containing ironas a main component Vinyl chloride-vinyl acetate   10 parts by weightcopolymer resin having a sodium sulfonate group Polyurethane resinhaving a   10 parts by weight sodium sulfonate group Polishing agent(AKP-30)   10 parts by weight Carbon black (#3250B)  1.0 parts by weightLubricant (myristic acid: butyl  3.0 parts by weight stearate = 1:2)Hardening agent  5.0 parts by weight (polyisocyanate) Cyclohexanone 65.8 parts by weight Methyl ethyl ketone 164.5 parts by weight Toluene 98.7 parts by weight

[0319] The viscosity of the obtained magnetic coating composition was7,680 cP.

[0320] The magnetic coating composition obtained was applied to thenon-magnetic undercoat layer obtained in Example 1 to a thickness of 15μm by an applicator, and the magnetic recording medium obtained wasoriented and dried in a magnetic field, and then calendered. Themagnetic recording medium was then subjected to a curing reaction at 60°C. for 24 hours, and thereafter slit into a width of 1.27 cm (0.5 inch),thereby obtaining a magnetic tape. The thickness of the respectivemagnetic recording layer was 1.0 μm.

[0321] The coercive force of the magnetic tape produced by forming amagnetic recording layer on the non-magnetic undercoat layer was 155.7kA/m (1956 Oe), the squareness (Br/Bm) thereof was 0.87, the glossthereof was 212%, the surface roughness Ra thereof was 6.0 nm, theYoung's modulus (relative value) thereof was 127, the linear absorptioncoefficient thereof was 3.86 μm³¹ ¹, the surface resistivity thereof was4.1×10⁵ Ω/cm², and the friction coefficient thereof was 0.21.

[0322] Core Particles 2

[0323] The same procedure as defined in Example 1 was conducted by using20 kg of the diaggregated acicular hematite particles (core particles 1)and 150 liters of water, thereby obtaining a slurry containing theacicular hematite particles. The pH value of the obtained re-dispersedslurry containing the acicular hematite particles was adjusted to 10.5by using sodium hydroxide, and then the concentration of the solidcontent in the slurry was adjusted to 98 g/liter by adding waterthereto. After 150 liters of the slurry was heated to 60° C., 5444 ml ofa 1.0 mol/liter NaAlO₂ solution (corresponding to 1.0% by weight(calculated as Al) based on the weight of the acicular hematiteparticles) was added to the slurry. After allowing the obtained slurryto stand for 30 minutes, the pH value of the slurry was adjusted to 7.5by using acetic acid. After further allowing the resultant slurry tostand for 30 minutes, the slurry was subjected to filtration, washingwith water, drying and pulverization, thereby obtaining the acicularhematite particles whose surface was coated with hydroxides of aluminum,(average major axis diameter: 0.144 μm; average minor axis diameter:0.0211 μm; aspect ratio: 6.8:1; geometrical standard deviation of majoraxis diameter: 1.38; BET specific surface area value: 55.1 m²/g;blackness (L* value): 28.5; volume resistivity: 4.8×10⁸ Ω·cm andmyristic acid absorption: 0.47 mg/m²). The amount of aluminum compoundcoated on the hematite particles is 0.98% by weight (calculated as Al)based on the weight of the hematite particles.

Examples 4

[0324] The same procedure as defined in Example 1 was conducted exceptfor varying kinds of core particles, kinds and amounts of polysiloxaneadded, edge runner treatment conditions used in the polysiloxane-coatingstep, kinds and amounts of the carbon black fine particles added, andedge runner treatment conditions used in the process for forming thecarbon black coat, thereby obtaining non-magnetic acicular blackiron-based composite particles.

[0325] Meanwhile, carbon black fine particles were added in three lotsin which the amount of each lot thereof was 10.0 parts by weight basedon 100 parts by weight of the core particles.

[0326] As a result of the observation by an electron microscope, carbonblack were not recognized in the non-magnetic acicular black iron-basedcomposite particles obtained. Therefore, it was confirmed that asubstantially whole amount of the used carbon black contributed to theformation of the carbon black coat on the coating layer composed of anorganosilane compound produced from the polysiloxane.

[0327] Various properties of the carbon black fine particles B and Dused, are shown in Table 4. The main production conditions are shown inTable 5, and various properties of the obtained non-magnetic acicularblack iron-based composite particles are shown in Table 6.

[0328] Meanwhile, “TSF484” (tradename, produced by GE TOSHIBA SILICONECo., Ltd.) was methyl hydrogen polysiloxane.

Example 5

[0329] <Production of Non-magnetic substrate: Formation of Non-magneticUndercoat layer on Non-magnetic Base Film>

[0330] By using the non-magnetic acicular black iron-based compositeparticles obtained in Example 4, non-magnetic undercoat layers wereformed in the same way as in Example 2.

[0331] The main producing conditions and various properties are shown inTable 7.

Examples 6

[0332] <Production of Magnetic Recording Medium: Formation of MagneticCoating Film>

[0333] Magnetic recording media were produced in the same way as inExample 3 except for varying the kind of non-magnetic undercoat layerand the kind of magnetic particles.

[0334] Various properties of the magnetic particles (1) used, are shownin Table 11.

[0335] The main producing conditions and various properties are shown inTable 8. TABLE 1 Properties of acicular he- Core particles matiteparticles or acicular iron oxide hydroxide particles Kind Particle shapeCore particles Hematite particles Acicular 1 Average Average Geometricalmajor axial minor axial Aspect standard diameter diameter ratiodeviation (μm) (μm) (−) value (−) Core particles 0.143 0.0210 6.8:1 1.381 Volume BET specific resistivity surface area Mn content value (m²/g)(wt. %) (Ω · cm) Core particles 55.3 — 2.3 × 10⁸ 1 Blackness Myristicacid (L* value) absorption (−) (mg/m²) Core particles 28.3 0.41 1

[0336] TABLE 2 Surface-treatment step Kind of Additives Core coreCalculated Amount particles particles Kind as (wt. %) Core particlesCore Sodium Al 1.0 2 particles aluminate 1 Surface-treatment stepCoating material Core Calculated Amount particles Kind as (wt. %) Coreparticles A Al 0.98 2

[0337] TABLE 3 Core particles Properties of surface-treated coreparticles Average Average Geometrical major axial minor axial Aspectstandard diameter diameter ratio deviation (μm) (μm) (−) value (−) Coreparticles 2 0.144 0.0211 6.8:1 1.38 BET specific Volume resistivitysurface area Mn content value (m²/g) (wt. %) (Ω · cm) Core particles 255.1 — 4.8 × 10⁸ Blackness (L* value) Myristic acid absorption (−)(mg/m²) Core particles 2 28.5 0.47

[0338] TABLE 4 Kind of carbon black fine particles Properties of carbonblack fine particles Average particle Geometrical standard Particle sizedeviation shape (μm) value (−) Carbon black B Granular 0.022 1.78 Carbonblack D Granular 0.030 2.06 BET specific surface area (m²/g) pH value(−) Carbon black B 133.3 3.4 Carbon black D 84.6 8.0 DBP oil absorption(ml/100 g) Blackness (L* value) (−) Carbon black B 84 14.6 Carbon blackD 95 17.0

[0339] TABLE 5 Production of non-magnetic acicular black iron-basedcomposite particles Coating with alkoxysilane or polysiloxane AdditivesAmount Kind of added core (part by Examples particles Kind weight)Example 1 Core Methyl triethoxysilane 2.0 particles 1 Example 4 CoreTSF484 1.0 particles 6 Production of non-magnetic acicular blackiron-based composite particles Coating with alkoxysilane or polysiloxaneCoating amount Edge runner treatment (calculated Linear load Time as Si)Examples (N/cm) (Kg/cm) (min.) (wt. %) Example 1 588 60 30 0.29 Example4 588 60 30 0.44 Production of non-magnetic acicular black iron-basedcomposite particles Adhesion step with carbon black coat Carbon blackAmount adhered Examples Kind (part by weight) Example 1 D 25.0 Example 4B 30.0 Production of non-magnetic acicular black iron-based compositeparticles Adhesion step with carbon black coat Amount adhered Edgerunner treatment (calculated Linear load Time as C) Examples (N/cm)(Kg/cm) (min.) (wt. %) Example 1 588 60 30 19.92 Example 4 588 60 3022.96

[0340] TABLE 6 Properties Examples of non-magnetic acicular blackiron-based composite particles Average major Average minor axial axialGeometrical diameter diameter Aspect standard deviation (μm) (μm) ratio(−) value (−) Example 1 0.145 0.0212 6.8:1 1.38 Example 4 0.145 0.02136.8:1 1.38 Volume BET specific surface area (m²/g) resistivity value (Ω· cm) Example 1 56.3 6.2 × 10¹ Example 4 55.9 4.4 × 10¹ Black-ness (L*value) Myristic acid Carbon black (−) absorption (mg/m²) desorptionpercentage (%) Example 1 17.8 0.21 6.6 Example 4 18.3 0.20 6.1

[0341] TABLE 7 Production of Properties of non-magnetic coatingnon-magnetic composition coating Kind of non- Weight ratio compositionmagnetic of particles Viscosity Examples particles to resin (−) (cP)Example 2 Example 52 5.0 374 Example 5 Example 53 5.0 415 Properties ofnon-magnetic undercoat layer Surface Young's roughness modulus ThicknessGloss Ra (relative Examples (μm) (%) (nm) value) Example 2 3.5 196 6.6123 Example 5 3.4 195 6.4 124 Properties of non-magnetic undercoat layerLinear absorption Surface resistivity Examples (μm⁻¹) value (Ω/cm²)Example 2 3.44 43 × 10⁶ Example 5 3.27 1.2 × 10⁶

[0342] TABLE 8 Production of magnetic recording medium Kind of non-magnetic Kind of Weight ratio undercoat magnetic of particles Exampleslayer particles to resin (−) Example 3 Example 54 Particles 5.0 obtainedin Example 3 Example 6 Example 55 Magnetic 5.0 particles (1) Propertiesof magnetic recording medium Thickness of magnetic Coercive Squarenesslayer force value Br/Bm Gloss Examples (μm) (kA/m) (Oe) (−) (%) Example3 1.0 155.7 1.956 0.87 212 Example 6 1.0 165.8 2.083 0.87 214 Propertiesof magnetic recording medium Young's Surface modulus Linear roughness Ra(relative absorption Examples (nm) value) (μm⁻¹) Example 3 6.0 127 3.86Example 6 5.9 127 3.89 Properties of magnetic recording medium Frictioncoefficient Surface resistivity Examples (μm⁻¹) value (Ω/cm²) Example 30.21 4.1 × 10⁵ Example 6 0.20 2.8 × 10⁵

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic base film; a non-magnetic undercoat layer formed on saidnon-magnetic base film, comprising a binder resin and non-magneticacicular black iron-based composite particles; and a magnetic coatingfilm formed on said non-magnetic undercoat layer, comprising a binderresin and magnetic particles, said non-magnetic acicular blackiron-based composite particles having an average major axis diameter ofusually 0.011 to 0.35 μm comprising: acicular hematite particles oracicular iron oxide hydroxide particles; a coating layer formed on thesurface of said acicular hematite particle or acicular iron oxidehydroxide particle, comprising at least one organosilicon compoundselected from the group consisting of: (1) organosilane compoundsobtained from an alkoxysilane compounds, and (2) polysiloxanes ormodified polysiloxanes; and a single carbon black coat formed on atleast a part of said coating layer comprising said organosiliconcompound, in an amount of 21 to 50 parts by weight based on 100 parts byweight of said acicular hematite particles or acicular iron oxidehydroxide particles.
 2. A magnetic recording medium according to claim1, wherein said acicular hematite particles or acicular iron oxidehydroxide particles are particles having a coat formed on at least apart of the surface of said acicular hematite particles or acicular ironoxide hydroxide particles and comprising at least one compound selectedfrom the group consisting of hydroxides of aluminum, oxides of aluminum,hydroxides of silicon and oxides of silicon in an amount of 0.01 to 50%by weight, calculated as Al or SiO₂, based on the total weight of theacicular hematite particles or acicular iron oxide hydroxide particles.3. A magnetic recording medium according to claim 1, wherein saidmodified polysiloxanes are ones selected from the group consisting of:(A) polysiloxanes modified with at least one compound selected from thegroup consisting of polyethers, polyesters and epoxy compounds, and (B)polysiloxanes whose molecular terminal is modified with at least onegroup selected from the group consisting of carboxylic acid groups,alcohol groups and a hydroxyl group.
 4. A magnetic recording mediumaccording to claim 1, wherein said alkoxysilane compound is representedby the general formula (I): R¹ _(a)SiX_(4—a)  (I) wherein R¹ is C₆H5—,(CH₃)₂CHCH₂— or n—C_(b)H_(2b+1) —(wherein b is an integer of 1 to 18); Xis CH₃O— or C₂H₅O—; and a is an integer of 0 to
 3. 5. A magneticrecording medium according to claim 4, wherein said alkoxysilanecompound is methyltriethoxysilane, dimethyldiethoxysilane,phenyltriethoxysilane, diphenyldiethoxysilane, methyltrimethoxysilane,dimethyldimethoxysilane, phenyltrimethoxysilane,diphenyldimethoxysilane, isobutyltrimethoxysilane ordecyltrimethoxysilane.
 6. A magnetic recording medium according to claim1, wherein said polysiloxanes are represented by the general formula(II):

wherein R² is H— or particle, particle-, and d is an integer of 15 to450.
 7. A magnetic recording medium according to claim 6, wherein saidpolysiloxanes are ones having methyl hydrogen siloxane units.
 8. Amagnetic recording medium according to claim 3, wherein saidpolysiloxanes modified with at least one compound selected from thegroup consisting of polyethers, polyesters and epoxy compounds arerepresented by the general formula (III), (IV) or (V):

wherein R³ is —(—CH₂—)_(h)—; R⁴ is —(—CH₂—)₁—CH₃; R⁵ is —OH, —COOH,—CH═CH₂, —C(CH₃)═CH₂ or —(—CH₂—)_(J)—CH₃; R⁶ is —(—CH₂—)_(k)—CH₃; g andh are an integer of 1 to 15; i, j, and k are an integer of 0 to 15; e isan integer of 1 to 50; and f is an integer of 1 to 300;

wherein R⁷, R⁸ and R⁹ are —(—CH₂—)_(q)— and may be the same ordifferent; R¹⁰ is —OH, —COOH, —CH═CH₂, —C(CH₃)═CH₂ or —(—CH₂—)_(r)—CH₃;R¹¹ is —(—CH₂—)_(s)—CH₃; n and q are an integer of 1 to 15; r and s arean integer of 0 to 15; e′ is an integer of 1 to 50; and f′ is an integerof 1 to 300; or

wherein R¹² is —(—CH₂—)_(v)—; v is an integer of 1 to 15; t is aninteger of 1 to 50; and u is an integer of 1 to
 300. 9. A magneticrecording medium according to claim 3, wherein said polysiloxanes whosemolecular terminal is modified with at least one group selected from thegroup consisting of carboxylic acid groups, alcohol groups and ahydroxyl group are represented by the general formula (VI):

wherein R¹³ and R¹⁴ are —OH, R¹⁶OH or R¹⁷COOH and may be the same ordifferent; R¹⁵ is —CH₃ or —C₆H₅; R¹⁶ and R¹⁷ are —(—CH₂—)_(y)—; y is aninteger of 1 to 15; w is an integer of 1 to 200; and x is an integer of0 to
 100. 10. A magnetic recording medium according to claim 1, whereinsaid acicular hematite particles are acicular manganese-containinghematite particles.
 11. A magnetic recording medium according to claim1, wherein said acicular iron oxide hydroxide particles are acicularmanganese-containing goethite particles.
 12. A magnetic recording mediumaccording to claim 1, wherein the amount of said coating organosiliconcompounds is 0.02 to 5.0% by weight, calculated as Si, based on thetotal weight of the organosilicon compounds and said acicular hematiteparticles or acicular iron oxide hydroxide particles.
 13. A magneticrecording medium according to claim 1, wherein the thickness of saidcarbon black coat is not more than 0.06 μm.
 14. A magnetic recordingmedium according to claim 1, wherein said non-magnetic acicular blackiron-based composite particles have an absorption amount of myristicacid of 0.01 to 0.3 mg/m².
 15. A magnetic recording medium according toclaim 1, said non-magnetic acicular black iron-based composite particleshave an average minor axis diameter of 0.006 to 0.18 μm, an aspect ratioof 2:1 to 20:1, a BET specific surface area of 35 to 300 m²/g, ageometrical standard deviation value of the average major axis diameterof not more than 1.50.
 16. A magnetic recording medium according toclaim 1, which further comprises a gloss coating film of 130 to 300%, asurface roughness Ra of coating film of not more than 12.0 nm, a linearabsorption of coating film of 1.90 to 10.00 μm⁻¹, a surface resistivityof not more than 1×10⁹ Ω/cm², and a coefficient of friction of 0.05 to0.30.
 17. A magnetic recording medium according to claim 2, whichfurther comprises a gloss of coating film of 135 to 300%, a surfaceroughness Ra of coating film of not more than 11.5 nm, a linearabsorption of coating film of 1.90 to 10.00 μm⁻¹, surface resistivity ofnot more than 1×10⁹ Ω/cm², and a coefficient of frictiona of 0.05to0.30.
 18. Non-magnetic acicular black iron-based composite particleshaving an average major axis diameter of usually 0.011 to 0.35 μm,comprising: acicular hematite particles or acicular iron oxide hydroxideparticles; a coating layer formed on the surface of said acicularhematite particle or acicular iron oxide hydroxide particle, comprisingat least one organosilicon compound selected from the group consistingof: (1) organosilane compounds obtained from an alkoxysilane compounds,and (2) polysiloxanes or modified polysiloxanes; and a single carbonblack coat formed on at least a part of said coating layer comprisingsaid organosilicon compound, in an amount of 21 to 50 parts by weightbased on 100 parts by weight of said acicular hematite particles oracicular iron oxide hydroxide particles.
 19. Non-magnetic acicular blackiron-based composite particles according to claim 18, wherein saidacicular hematite particles or acicular iron oxide hydroxide particlesare particles having a coat formed on at least a part of the surface ofsaid acicular hematite particles or acicular iron oxide hydroxideparticles and comprising at least one compound selected from the groupconsisting of hydroxides of aluminum, oxides of aluminum, hydroxides ofsilicon and oxides of silicon in an amount of 0.01 to 50% by weight,calculated as Al or SiO₂, based on the total weight of the acicularhematite particles or acicular iron oxide hydroxide particles. 20.Non-magnetic acicular black iron-based composite particles according toclaim 18, wherein said modified polysiloxanes are ones selected from thegroup consisting of: (A) polysiloxanes modified with at least onecompound selected from the group consisting of polyethers, polyestersand epoxy compounds, and (B) polysiloxanes whose molecular terminal ismodified with at least one group selected from the group consisting ofcarboxylic acid groups, alcohol groups and a hydroxyl group. 21.Non-magnetic acicular black iron-based composite particles according toclaim 18, wherein said alkoxysilane compound is represented by thegeneral formula (I): R¹ _(a)SiX_(4−a)   (I) wherein R¹ is C₆H₅—,(CH₃)₂CHCH₂— or n—C_(b)H_(2b+1)—(wherein b is an integer of 1 to 18); Xis CH₃O— or C₂H₅O—; and a is an integer of 0 to
 3. 22. Non-magneticacicular black iron-based composite particles according to claim 21,wherein said alkoxysilane compound is methyltriethoxysilane,dimethyldiethoxysilane, phenyltriethoxysilane, diphenyldiethoxysilane,methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane,diphenyldimethoxysilane, isobutyltrimethoxysilane ordecyltrimethoxysilane.
 23. Non-magnetic acicular black iron-basedcomposite particles according to claim 18, wherein said polysiloxanesare represented by the general formula (II):

wherein R² is H— or particle, particle-, and d is an integer of 15 to450.
 24. Non-magnetic acicular black iron-based composite particlesaccording to claim 23, wherein said polysiloxanes are ones having methylhydrogen siloxane units.
 25. Non-magnetic acicular black iron-basedcomposite particles according to claim 20, wherein said polysiloxanesmodified with at least one compound selected from the group consistingof polyethers, polyesters and epoxy compounds are represented by thegeneral formula (III), (IV) or (V):

wherein R³ is —(—CH₂—)_(h)—; R⁴ is —(—CH₂—)_(i)—CH₃; R⁵ is —OH, —COOH,—CH═CH₂, —C(CH₃)═CH₂ or —(—CH₂—)_(j)—CH₃; R⁶ is —(—CH₂—)_(k)—CH₃; g andh are an integer of 1 to 15; i, j and k are an integer of 0 to 15; e isan integer of 1 to 50; and f is an integer of 1 to 300;

wherein R⁷, R⁸ and R⁹ are —(—CH₂—)_(q)— and may be the same ordifferent; R¹⁰ is —OH, —COOH, —CH═CH₂, —C(CH₃)═CH₂ or —(—CH₂—)_(r)—CH₃;R¹¹ is —(—CH₂—)_(s)—CH₃; n and q are an integer of 1 to 15; r and s arean integer of 0 to 15; e′ is an integer of 1 to 50; and f′ is an integerof 1 to 300; or

wherein R¹² is —(—CH₂—)_(v)—; v is an integer of 1 to 15; t is aninteger of 1 to 50; and u is an integer of 1 to
 300. 26. Non-magneticacicular black iron-based composite particles according to claim 20,wherein said polysiloxanes whose molecular terminal is modified with atleast one group selected from the group consisting of carboxylic acidgroups, alcohol groups and a hydroxyl group are represented by thegeneral formula (VI):

wherein R¹³ and R¹⁴ are —OH, R¹⁶OH or R¹⁷COOH and may be the same ordifferent; R¹⁵ is —CH₃ or —C₆H₅; R¹⁶ and R¹⁷ are —(—CH₂—)_(y)—; y is aninteger of 1 to 15; w is an integer of 1 to 200; and x is an integer of0 to
 100. 27. Non-magnetic acicular black iron-based composite particlesaccording to claim 18, wherein said acicular hematite particles areacicular manganese-containing hematite particles.
 28. Non-magneticacicular black iron-based composite particles according to claim 18,wherein said acicular iron oxide hydroxide particles are acicularmanganese-containing goethite particles.
 29. Non-magnetic acicular blackiron-based composite particles according to claim 18, wherein the amountof said coating organosilicon compounds is 0.02 to 5.0% by weight,calculated as Si, based on the total weight of the organosiliconcompounds and said acicular hematite particles or acicular iron oxidehydroxide particles.
 30. Non-magnetic acicular black iron-basedcomposite particles according to claim 18, wherein the thickness of saidcarbon black coat of not more than 0.06 μm.
 31. Non-magnetic acicularblack iron-based composite particles according to claim 18, whichfurther comprises an absorption amount of myristic acid of 0.01 to 0.3mg/m².
 32. Non-magnetic acicular black iron-based composite particlesaccording to claim 18, which further comprises an average major axisdiameter of 0.006 to 0.18 μm, an aspect ratio of 2:1 to 20:1, a BETspecific surface area of 35 to 300 m²/g, a geometrical standarddeviation value of the average major axis diameter of not more than1.50.
 33. A non-magnetic substrate comprising: a non-magnetic base film;and a non-magnetic undercoat layer formed on said non-magnetic basefilm, comprising a binder resin and non-magnetic acicular blackiron-based composite particles having an average major axis diameter ofusually 0.011 to 0.35 μm, comprising: acicular hematite particles oracicular iron oxide hydroxide particles; a coating layer formed on thesurface of said acicular hematite particle or acicular iron oxidehydroxide particle, comprising at least one organosilicon compoundselected from the group consisting of: (1) organosilane compoundsobtained from an alkoxysilane compounds, and (2) polysiloxanes ormodified polysiloxanes; and a single carbon black coat formed on atleast a part of the coating layer comprising the organosilicon compoundcoated, in an amount of 21 to 50 parts by weight based on 100 parts byweight of the acicular hematite particles or acicular iron oxidehydroxide particles.
 34. A non-magnetic substrate according to claim 33,wherein said acicular hematite particles or acicular iron oxidehydroxide particles are particles having a coat formed on at least apart of the surface of said acicular hematite particles or acicular ironoxide hydroxide particles and comprising at least one compound selectedfrom the group consisting of hydroxides of aluminum, oxides of aluminum,hydroxides of silicon and oxides of silicon in an amount of 0.01 to 50%by weight, calculated as Al or SiO₂, based on the total weight of theacicular hematite particles or acicular iron oxide hydroxide particles.35. A non-magnetic substrate according to claim 33, which furthercomprises a gloss of coating film of 170 to 280%, a surface roughness Raof coating film of 2.0 to 12.0 nm, a linear absorption of coating filmof 1.50 to 5.00 μm⁻¹, and a surface resistivity of 1×10³ to 1×10¹¹Ω/cm².
 36. A non-magnetic substrate according to claim 34, which furthercomprises a gloss of coating film of 175 to 280%, a surface roughness Raof coating film of 2.0 to 11.5 nm, a linear absorption of coating filmof 1.50 to 5.00 μm⁻¹, and a surface resistivity of 1×10³ to 1×10¹¹Ω/cm².